Ok a few thoughts here. I have a Merc/Traps MM whose's now in the mid 30's. (and a Zombie Dark in the 20's) While I do have a few issues with the current build I've not felt the need to respec it yet.
One thine I like about mercs over robots is that the Medic heals me as well as healing the other pets. He doesn't however heal himself so you need a way to keep him alive and medicine does that nicely.
Of the Mercs Pool I've taken everything bar the M30 Grenade, I felt that doing AOE's was just a way of attracting agro on me rather that having my pets take the damage. I may change this if I respec.
The one Power I've taken from the line that I just don't use is Serum. It doesn't seem to turn my Commando into a mega killing machine when I use it so it will probably go when I respec.
From Traps I've gone with Caltrops, Acid Mortar, Force Field and Drones. Not sure If I'll take the various traps or Detonate - seems foolish to kill your main attacking ability by bumping off your own minions - may take this before respecing just for kicks and Giggles. Traps could be useful if I use the Spec Ops sniper ability to pull mobs on to them, but I'm running out of slots for powers so not certain as yet. Plus groups don't usually want to wait while Traps get laid before pulling.
For the Pools
My initial power at lvl 6 was Medicine and I've never regretted taking it. Aid other, which does work on your minions, is much more useful than Beacon. which I've not taken, having spoken with a number of other MM's who did, and having seen it being used I think aid other is far better, although when you're getting continually interupted by AOE attacks it can be a pain. I've seldom missed having beacon. Probably won't take Aid Self as being interuptable its usually useless when you need healing most.
I now have all three basic leadershp powers and won't get vengeance. Running all three toggles is an endurance hog but slotting with End reducers deals with this. My intention with these was to slot two end reducers and then 1-2 in appropriate buffs, (though I'm currently running with 3 end reducers in Assault, which I'll probaly stick with) This seems to work reasonably well - though end was a problem until I managed to get the end reducers in.
I've taken stamina on just about every alt I've created except for my MM's. OK at the start of missions all the summoning/buffing is a real end drain and I usually have to kick a rest in, but that just allows the tactical mastery to recycle so its not too bad. During missions themselves I'm usually fine unless I get all excited and start blasting away, rather than controlling the pets.
My final pool power is Speed. I may respec out of this. I partially took this pool for hasten as having powers up faster gives me more options, and partially for SS itself. I don't find it a problem with leaving minions behind as I don't usually run it on missions and don't usually travel with minions in tow.
I spend most of my time on missions directing the minions so I'm not so certain Hasten is needed. It could be replaced by dropping a few recharge enhancers into one or two powers.
