Pretty much no set bonuses apply to pets, just to the mastermind. The only reason to slot full sets of pet damage IO' s is if you are chasing after def for your MM and want the melee def in Sovereign Right, or the AoE and Ranged def in blood mandate, which on a traps mastermind who wants to act as a tank in BG mode is not unreasonable. The exception is the pet aura IO's (+def and +resistance) - and those are not really an exception, they don't grant the pets anything they give YOU a bonus aura that works a lot like supremacy, its that aura that affects the pets.
Other than that you are best off frankenslotting the pets. With level 30 IO's you can cap damage and get over 60% accuracy in just 4 slots (3 x Acc/Dam, 1x Dam) or you can slot for some end reduction (which will affect the pet attacks) and get something like 56% accuracy, 88% damage and 34% end reduction (2 x acc/dam/end, 1x acc/dam, 1x dam). Then you can use the remaining two slots for procs, if the pets have attacks that will take them, pet resist IO's or non-damage IO's. For example, slotting extra DEF in thugs enforcers is good because it enhances their maneuvers - you actually get double benefit from this since there are two of them.
I think the builds that have full sets are either chasing after def bonuses for the MM or folks are just slotting full sets out of habit.
Globals: @Midnight Mystique/@Magik13
Also some Pet sets give decent +END and +RECOVERY. While you don't spend end on your damage (minions take care of that) some secondaries bleed chunks of you blue bar. Lightning Storm costs close to a whopping 40 END!!!! Re-summoning minions in combat can cost a hefty chunk of blue as well. You can also grab some decent +REGEN from pet sets, to help with say Pain Domination.
Really depends on how you want to play it. You may get less from a global damage bonus but sometimes that damage bonus is just part of a set you want another bonus (DEF/RECH/ETC.) from.
Thanks for the info!
I have seen builds full of sets that increase the damage and defense, but is this just to the Mastermind himself or to the pets or both? Since the mastermind usually isn't a target they really don't need defense themselves. My thinking is that if a brute doesn't add defense to their attacks, why would it add defense to a pet (which as you know is an attack)?