That seems to be a close to universal opinion, which is why it is usually recommended to be skipped. I have heard that some Merc/Pain MMs take it because the Pain buffs can supposedly negate the crash.
Arc #40529 : The Furies of the Earth
I might argue that Serum is there to give Mercs (a ranged MM set) the ability to microtank against an AV or GM. Slap it on your Commando, send him off on aggressive to a target to build hate, and then bring in your other pets with spread heals once everything is seated.
Honestly though, the super long recharge ultimately makes Serum an 'Oops' button. If you pull an extra group, run into an emergency, or have an important pet who's about to eat it who you REALLY need, then Serum fits the bill nicely.
Is it skippable for most set pairings? Absolutely. If you have a secondary with limited or no healing, however, and rely on debuffs and other tricks to keep yourself going, it could be worthwhile. This is one of those powers where mileage varies greatly from secondary set to secondary set, and player to player.
Do remember that slotted and with enough set bonuses and hasten, you can get the recharge down to 4-5 minutes. Something that makes a pet that much harder to kill, fills their endurance bar, and makes them virtually immune to mez can potentially be worth it, but if you're already using a set that performs these functions, I'd agree it's mostly pointless.
Ok...you will probably laugh at me, but I really don't get it.
I have recently gotten my mercs/traps MM to 18 and I thought that the recharge listed for Serum was incorrect. Apparently it's not...
16 minutes??? Why? Why not just bring the resists down to reasonable numbers and half the recharge time? That's just nuts. Or am I missing something incredible with regards to it. Compared to all the other MM level 18 powers...this seems like mulch.