Interesting point, well argued.
I have to admit that as enlightened a guy as i think I am... I have a hard time believing the female character's voice actor to be the toughened yet stoic veteran of interstellar wars that the male Shepard conveys.
Perhaps that's a personal preference.
Also... there's something about the actual voice itself that kinda turns me off. This may have been a choice by the actor, but it seemed sort of monotone and flat...
Perhaps to make her character seem tougher, masculine? I'm not sure it worked for me.
I think that while her interpretation as Shepard was no doubt a very good one, perhaps it really shone in the quiet human moments than in the confrontational or persuasion necessary ones.
For me, my Shepard was a charismatic soldier-diplomat type, who reasoned when necessary, but stuck the boot in when all else failed.
If he didn't shine as much in the human moments then I reason it to be just a sign of a toughened military man.
Emotionally in control, but guarded. After all he's seen and done in the galaxy, it's just part of how he operates, it's not a failing at all.
So, while I see your point, I don't think I would have believed a female Shepard nearly as much as in a male one.