Oh dear god, no. Not that stupid mode dial unlock button.
You obviously don't spend a lot of time with a camera hanging by your side.
I do. Hours and hours walking around streets with my 5DII hanging at my side, or hours and hours hiking through the wilderness with my 7D hanging at my side.
Number of times I've accidentally rotated a mode dial? Perhaps 3-4.
Number of missed shots resulting from that? One - the first time.
Number of times I've looked up to see an action scene unfolding on front of me or a bird flying close by, and been able to quickly spin the mode dial to the hard stop at C3 to get the shot? Probably in the hundreds.
Number of missed shots resulting from having to press a button before spinning a dial? I don't know, but I can almost guarantee it'll be more than one.
It's really not that hard. You don't use an extra hand and the other finger that is required is generally not far away (for most people, anyway.)
Do you have a 60D or mode dial-converted 7D or 5DII that you've done this with? Currently, the camera hangs at my right hip on a BlackRapid strap. I grab it with my right hand and as I'm bringing it up in front of my body, I rub the mode dial across the palm or fingers of my left hand as that hand reaches for the lens barrel. It's one smooth, fast motion, and I can't see how having to use a thumb or finger to press a button and then spin the dial can be as smooth or as fast.
OTOH, I run into this mostly with the 7D and birds. So, in a 5DII replacement it probably isn't a big deal. Personally, considering that I'm planning to get a 1D X with no dial at all, changes are in order for me, regardless.
I can't see why Canon didn't just design a seperate lock switch that you could turn on or off instead of the 'push and turn' design. That way if you don't want to lock the mode dial, you don't have to use it.