If you were not sufficiently slotted for accuracy before, then yes I can see it making a big difference in performance. I wouldn't expect it to make the difference between face-planting or not though; yes killing foes faster will mean you're taking less damage, but with enforcers spraying -def lead around, hitting things is rarely an issue on my thug/. Are you sure you weren't using the radial version of the boost? That would be much more likely to affect survivability, assuming your pets weren't soft-capped already.
If you were underslotted on accuracy for some of your powers, then I can well imagine that Nerve makes a big difference.
For myself, I didn't see much of a difference with Nerve until I got to my Nerve Radial - my Protector Bots and a couple of other powers were not slotted at all for +Def so the 20% defence actually ended up putting me above softcap for Smashing/Lethal. You may find the same if you've not slotted your Enforcers for +Def
My Musculature Core likewise was noticeable for the unenhanced immobs I had.
The Alpha Slot is all about shoring up your weaknesses - pick wisely 
I don't have 2nd Tier Nerve Radial yet, so I know that didn't contribute, but I had very good defense already (via IO slotting, having all special MM def/resist uniques slotted in gangwar, plus Personal Force Field and Maneuvers running)
My pets had decent accuracy already (Thugs had Soulbound Allegiance full purple set, other pets have full IO sets as well), plus I run Tactics.
Here's a sample number: Leiutenant Accuracy without Nerve: 1.62. With: 1.92
Yeah, I wouldn't have thought the above would have made that huge a difference either, but I know the difference between personally faceplanting twice and completing the same mission with only thugs dying a few times. *Shrug* I dunno... like I suggested, maybe the programming dice just rolled well for me.
My MM can cruise thru just about anything at +8, but those Arachnos were giving me some problems (and Peacebringer mobs do as well *snicker*). I'll try these a few more times to get a good feel.
Regardless, due to pets conning lower than just about everything they fight, I suggest Nerve Incarnate slot to any MasterMind and give them some Accuracy to offset it.
Your pet's accuracy was probably quite under-slotted. Most MMs don't usually have accuracy problem as Supremacy gives tohit buff and a lot of MMs run leaderships.
Yeah, when you are under-slotted for certain enhancement, Alpha definitely makes a huge difference. That's why Muscular feels like it's not giving enough +damage because most sets already push Damage enhancement so high and you only get the bypass value which isn't much on pets with low base damage.
My Bane took Nerve and I can't wait to try 2nd tier with more defense. Seriously, Nerve is really only good for the +defense. My Bane runs Tactics already and there's just not many things I need extra tohit.
What's left is to normalize all Assassin Strikes and improve Stalker's old sets (Claw, MA and EM)! You don't need to bring back the missing PbAoE attack. You just need to make the existing ones better! For example, make Slice a WIDER and LONGER cone.
To be honest, I am not sure which mastermind primary you are running. Moving from ~80% to 95+% accuracy can be a big deal for lowest and mid-tier pets. For instance, if they player is running a robots, thugs, or demon set which needs to address endurance reduction, defense, resistance, extra accuracy for the second tier guys, enforcers, ember demons, gargoyles, or protector bots can be simply huge.
Somebody can do the numbers, but if the mastermind is +1, +2, running leadership on himself or enforcers, the boost of 15-20% accuracy modified by tactics is even more noticeable for the little guys.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
So, I just recently got my thugs/traps MM to 50, and I could plug along at +8 fairly well against most enemy types, but Arachnos was giving me fits. I had this mish in RWZ full of Arachnos that was causing me more than one hospital visit (and I refuse to lower the difficulty - sometimes overcoming a challenge is part of the fun).
So, mid mish I get a Shard Drop that enables me to convert for the last piece to get my Tier 1 Incarnate. I figure the Nerve would boost the accuracy of my Pets, so I immediatley select that and slot it, and look at my enchancement window to see the results - it was a pretty significant jump.
So, I reset the mish to see what difference it would make. And what a difference - I slaughtered those Arachnos like it was spider hunter season! I seriosly couldn't believe an global enhanced accuracy boost would make that huge of a difference, although with Tier 1 pets at level 48 and Teir 2's at level 49 I guess an accuracy boost to better hit higher conning mobs might make some sense. But that much...wow!
I'm gonna test this some more, but basically I'm wondering if it's really that good or if I just had a lucky mish run. Seriously, Teir 1 Nerve seemed to turn to the mish from very difficult to a typical one, bordering on easy. Has anyone else experienced this jump in effectiveness by having Nerve slotted?