The way I see it: If I go for a left-handed mouse, what happens when I'm at a friend's house, or a library, or ANYWHERE, and I encouter a mouse? 9 times out of 10 (literally!) it'll be right-handed. But whatever. I'm also a big fan of Logitech (the g5 has lasted me way beyond it's price, some of the best 45 dollars I've ever spent on hardware.
You can use a mouse with either hand, it's a matter of using the superior hand when you can. Certainly you don't need the same level of control to browse the internet of look up call numbers as you would for gaming... plus almost all non high performance mice, which you will find most everywhere, are not ergonomically designed, and as such are equally suited to either the left or right hand, so there's no reason you can't just move it over to the other side. I mean really... when was the last time you saw an mx518 on the card catalog station at your library?
I have superior control in my right hand (My left hand still has a power advantage), probably because I've trained it for so long on right-handed mice. I've had a G5 for about 2 years now, and it is DEFINITELY righties-only. So, really, there's several reasons:
1) I don't want to blow another 50 dollars on another gaming mouse when I have one of the best ever made right here
2) I don't want to learn to switch
3) Edit: The only leftie/ambidextrous gamer mouse I can find is the G3. I've never had a good experience with ambidextrous mice.