Uhm, the system is not limited to +/-1. There's no cap, as has been pretty well demonstrated by various people in various threads. Unless I missed a memo from PW. I'm pretty sure Saint has never rolled anything aside from AUX:5's.
And yes, almost every fleshtech has significant medical training in their backstory. Previously, a fleshtech's... variable performance was usually handwaved as "You're in a panicked, stressful situation, rather than a nice clean surgical operating room with several helpers and plenty of time. You'll make mistakes." It doesn't make sense that a doctor with plenty of time to prepare can't get a guaranteed five. Or a sniper. Or a mechanic. It makes even less sense that the person only has a one in six chance of not making some form of mistake.
Also, why would Xan/whoever being able to get guaranteed fives on uncon rolls be bad? He could buy two freaking Avatars of War for the tokens he'd need, plus many missions to get the stats, and he still wouldn't stand a chance against, say, the AM. And being able to roll perfectly does not equate to being invincible. He can still make bad decisions, and he can always get into situations that just kill you with no roll.
NAV, I really dislike that argument. When has charging for dynamic bonuses slown anything down? I mean, yes, in theory it could become a problem. But it isn't. Why change things to fix a 'might'.
Lastly, I do agree that it shouldn't be possible to just charge a generic bonus. I think we should go back to the original design, which was "do action which prepares for this action". If you can't specify what action you're preparing for, or if it's impossible for you to prepare, then you can't get a dynamic bonus.