Storm, Traps, and FF deal with it by keeping things from hitting the pets enough to kill them. 
As for TA, its debuffs are AoE so it's easier to deal with a horde. Poison is one of the strongest secondaries against a single hard target, but it's the weakest against groups.
Also, your Thugs have a fair amount of defense from the Enforcers' leadership buffs while the Mercs have none. That means they get hit more, and need more healing. Thugs / Poison would be a lot sturdier than Mercs / Poison since you have defense plus a nice tough Bruiser to attract aggro off your squishy tier one pets.
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
The Instant Lair Delivery Service: 206636
Pff, the Arsonist draws way more aggro than the Bruiser. My Commando is much better at keeping the aggro off the lower tier pets than the Bruiser is.
Issue 16 made me feel like this.
Warning: This poster likes to play Devil's Advocate.
It comes down to the pets not the matter of alkaloid being awful. Alone Thugs survive much better than Mercs, poison while powerful doesn't have much mitigation. Both mercs and thugs are heavy on the AoE so they get aggro very easily.
Having a Merc/Pain I can tell you they needed every bit of heal I could output. However my Thug/Poison does just fine keeping his pets alive with just alkaloid. A Thug/Pain is much better geared for fights vs large spawns. MMs in general do well vs just one target.
My Necro/Storm does fine with just O2 because of all damage mitigation tools the set provides. My Thug/FF has enough defense that aid other is all he needs for healing.
Take into account Masterminds are unlike the other AT in this game. To the fact changing either the primary or secondary changes playstyle completely. My Thug/FF plays nothing like my Thug/Poison. I'll be starting a Necro/Traps soon and I'm sure it will play nothing like my Necro/Storm.
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Pff, the Arsonist draws way more aggro than the Bruiser. My Commando is much better at keeping the aggro off the lower tier pets than the Bruiser is.
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I don't count the Arsonist... he's doomed even if you run a co-op TF with seven Tankers. 
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
The Instant Lair Delivery Service: 206636
[ QUOTE ]
Some might say that the debuffs from Poison make up for it. While that's more or less true against single tough foes, the fact is that you just can't debuff multiple enemies fast enough to make up for it. In addition, in order for the debuffs to really matter, you've gotta have Noxious Gas on them. I can't tell you how many times I've fought bosses pre-38 or when NG isn't up and stuck them with Weaken and watched them casually kill all my pets anyway. Sure I can heal or two of them with Alkaloid, but by then the boss's next AoE volley is up and all the other pets are dead.
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I have a level 34 Merc/Poison, and I'd agree that there are times when my pets die ALARMINGLY fast. Like, facing Longbow Flamethrowers. Talk about melting.
Bosses are usually fine, as long as I grab aggro first and do my darnedest to keep it. I try this by teleporting just to the other side of the boss, getting Envenom queued up during the TP. Then I continue with Slug, Weaken, Neurotoxic Breath, etc while constantly positioning to keep the boss between me and my minions. Usually works. But yeah, if it goes bad, it goes bad - and Alkaloid isn't going to be enough to help. At that point I just try to resummon best I can while trying to keep the Commando on his feet.
Suggestions:
Super Packs Done Right
Influence Sink: IO Level Mod/Recrafting
Random Merit Rolls: Scale cost by Toon Level
Noxious gas is the best power in /poison, so using it with mercs/ will be hard since they aren't melee. Thug/psn works better.
Anything with melee works well with /Poison.
Ninjas, Zombies, etc.
I think it actually works best with thugs. The high heal/sec heal works great on single targets taking a lot of damage (bruiser). And the modifier changes depending on which pet you use it on so if you use it on the tier 3 (the only tier 3 that is melee is bruiser) it will be a better debuff. Also, neurotoxic breath or any long range permanent slow works great for keeping enemies out of range of your ranged pets (which is why merc/TA works so well).
Time for me to express my hate for /Poison! >
Remember that Poison is the stronger Single target debuff because is the only freaking secondary that have his biggest debuffer on his single targets power. His only 2 AoE aren't so amazing since one is a cheap slow cone and the other have a long recgh.
We are Mastermind,We are CoV tanker,We don't waste our precious time alone in a missh fighting one target at the time, that is a stalker job.Our purpose is to be able to kill anything that dare to go against our army is the most efficient/faster way.
Poison can't keep a pet alive in a normal size team,It need two heal application to heal a minion pet.Also,Poison have only been say great when pair with Thug.Can't get why ppl always praise that combo...what is so great when a powerset can only be great pair with one specific power?
Storm,Dark,Pain,Trap and even FF have the advantage that will performance excellent whichever set you decide to pair him.
Poison MM should stay in PvP zone...
I want /Fire stalker. Because nothing says stealth like dumping a can of gasoline on yourself and lighting a match. -Morac
Okay, so recently I got the urge to make a Thugs/Pain MM. This MM naturally has Nullify Pain and Soothe. I've played this dude up to level 34 so far. Now, the other day I get the urge to play my 46 Merc/Poison MM. And the difference is night and day. With my Pain MM, I just constantly use NP to keep my Thugs in tip-top shape with the occasional Soothe to save one in big trouble. With my Poison MM, virtually every fight I start begins with one of my Mercs nearly dying. I've been in situations with both where aggro is out of control enemies are all over and I can't direct my henchmen because I can't leave Bodyguard mode. With the Pain, I'm spamming NP and Soothe, and 9 times out of 10 I win the fight with maybe a few dead pets. With the Poison I desperately try to heal my men but with the long time between activation and healing being delivered, the pathetic values on the heal, and the high end cost, I end up with no endurance, a bunch of dead pets, and the need to retreat.
Some might say that the debuffs from Poison make up for it. While that's more or less true against single tough foes, the fact is that you just can't debuff multiple enemies fast enough to make up for it. In addition, in order for the debuffs to really matter, you've gotta have Noxious Gas on them. I can't tell you how many times I've fought bosses pre-38 or when NG isn't up and stuck them with Weaken and watched them casually kill all my pets anyway. Sure I can heal or two of them with Alkaloid, but by then the boss's next AoE volley is up and all the other pets are dead.
In short, I've never once wanted for debuffs on my Pain MM, but nearly every time I've played my Poison I've desperately wished I had an AoE heal. I can't imagine how Storm, TA, Traps, and FF deal with it, but I'd imagine that the AoE debuffs or solid pet buffs are a great help. I wish Alkaloid were an AoE, with no other changes to the power.
Issue 16 made me feel like this.
Warning: This poster likes to play Devil's Advocate.