There are various opinions given on this in the articles linked to in the Mastermind Index (first post stickied at the top of this forum).
Which isn't to say other people might not give their opinions in this thread too. 
It depends.
A Blastermind may need stamina or conserve if they're attacking continually.
Normal MM's don't really need it; but if you're PvP'ing, it may be necessary to get or carry a lot of Blues.
Stamina... for a Mastermind its something you need to respec into if you find you need it. But consider, you lose three other powers to gain it. Is it really that important to you?
If so, great! If not, then you can work without it.
I can run five toggles simultaneous and still heal/attack. I can run another toggle on top of that for the duration of a fight if needed. Without Stamina.
Of course, you want to spam something over and over that needs more slotting then yes - Stamina could be the way to go!
I played my MM very activly with alot of holds/debuffs/fears going all the time. Maybe I didn't have too but that was the playstyle for me.
If I instead just stood back and did the occasional heal I would have survived fine without stamina, did so for 30 levels before I got it. That kind of playstyle got really boring though, felt like I was watching someone else play instead of actually participating so I went for stamina and a more active role.
It also helps alot if you need to resummon mid fight in say an AV fight.
If you think that you need it, then you can get it, thats all there is to it really. Some people get it, some dont. My robo/poison needs it but same need not apply to you.
I'm quite happy I took it on my Thugs/Dark since it allows me to run several toggles at once as well as spamming debuffs/heals (and re-summoning henches anytime it's needed) without having to worry about endurance.
CoX 50s: <ill/rad> <ice/ice> <fire/kin> <grav/sonic> <ice/storm> <earth/kin> <kin/elec> <cold/psy> <thugs/dark> <fire/dark> <dark/elec> <night widow> <EM/ninj> <mind/icy>
I have a thugs/dark and even me picking all the secondary set bar black hole and the nessaries from primary set I have 5 free slots.
Which I used on a travel power and swift/health/stamina.
This does not include lvl 41+ so you cant either get your ppp or other power pool set which means even more open choice got to love MM's
recently got right back into playing my MM after hitting the evil lvl 30 wall, and tbh i just respeced into stamina as a Necro/Dark MM i dont think i could live without Stamina with all my holds and fears and heals going off left right and center
Necro/Dark MM speaking:
I run very well without... when things go smoothly. I throw in my toggles, a few holds/fears and I end the fight with nearly full endurance.
However, I find myself in dire need of Stamina sometimes... mostly when someone (a pet or a teammate) brings additionnal aggro or when I'm soloing an EB (didn't try soloing an AV yet). The latter case I can win (depends on the Boss: I can do nothing against Infernal or other fire-based boss), but I use 2-3 end inspirations on the way.
I already intend to put Stamina in my build, though I'm not in any hurry.
Infernal is indeed a hard nut to crack. I tried twice on relentless. He was then stuffed as a Hero and he was still at 60% of health after he finishes off all my pets (including a summoned Shiva). I run a robo/FF with stamina. I have a few endurance heavy toggles and well being able to keep a boss on the ground with forcebolt is great and only with stamina I can do that.
I have both a bots/ff and a thugs/dark..neither have any fitness.
No end probs on the bots mm even with 6 toggles going..but i only have sands of moo for an attack so apart from initial setups, no end probs at all. Just need a few end reducs 
Necro/Poison MM here:
Only level 17 at the moment, but I think I can live without it, yes I need an endurance reduction or two on some of my very common or particularly draining powers, but I feel once I'm slotted for DOs I'll only need one, and that's par for the course for any power you use a lot, ask a Scrapper.
Professor Lente is my Robo/Poison MM (lvl42 atm) and I certainly wouldn´t leave home without stamina. I still drain my endurance regularly.
I guess it all depends on the playstyle. Some like to be rather passive MMs and/or have secondary sets with low endurance consumption. Others play a more active role, at least against tougher opponents, and will easily use enough endurance to justify stamina.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
Merc/TA, no Stamina, TA is a pretty end-friendly set. Though i have no toggles running.
Robot/poison, have Stamina, healing power and the single target debuffs can drain it fast and having 2 leadership running.
thugs/dark, planning to go Stamina. Mainly due the toggles and trigger happy powers.
Im a quite active MM, spamming powers nonstop and use my (only 1) attack to help ditching out some extra damage. Specialy with the massive DoT mercs do it sometimes can bit somewhat annoying.
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
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Do Masterminds really need Swift or Hurdle, Health and Stamina?
I mean really, we aren't going to be taking damage as our henchmen will be doing the damage and taking it and we will also be buffing/debuffing instead of attacking. Do we really need them as I am puddled on whether we do or don't 
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