All you have to do is try to take a Mastermind into Incarnate level content and see how badly they perform to settle pretty much every argument in this thread.
That's using supposedly "strong" MM sets like Demons, Bots, and Thugs, as well. Let's not even talk about Zombies or *cough*Ninjas*cough*.
Yes, pets are ridiculously strong at low levels, but that's counterbalanced by the fact that we don't get many pets at low levels. At high levels, the level penalty to pets, and the other endurance and damage penalties stacked upon the MM specifically, plus the fact you can't share set bonuses between the pets and the MM, mean you're effectively crippled. Additionally, the fact that if your pets are killed (which they will be, constantly) you have pretty much nothing left to go on as far as a traditional MM approach and have to wait to re-summon them (if you aren't killed first) means you suffer from a lot more downtime than you would if you were another AT facing a simple defeat.
I'm speaking as someone who loves Masterminds, whose Main is in fact a Mastermind, who is proud of the AT and doesn't want to admit any weaknesses that it has. And yet I'm forced by gameplay to admit that my TANKER performs better in high-level and especially Incarnate content, in every single aspect, than my beloved MM main.