I've got a couple of a/b/g cards. I don't think I've ever really needed the "a" part. I did setup an adhoc network using "a" one time, but I really didn't see that much of a speed increase. It might have been a driver limitation, though.i am hoping for a a/b/g card but i will keep a look out for this one.
"A" is pretty short ranged and for general usage and pentesting, I doubt you'll ever need it. I'm waiting for "n" to become standardized before upgrading anymore of my cards. Ought to see some funny posts here, then.