A mic is a nice supplement to the keyboard. I've always typed fast, so I never really needed one. Even back in games like XvT or XWA folks would say "wow you type fast!" (no vox in those games, you see). However, a mic is a good efficient way to communicate. Instead of hitting auto level, hitting "/" and then typing "which way did those bombers go?" hoping somebody reads the buffer on range, you could just press a button and say the same thing.
There are pros and cons with everything, but I think it's good to have, especially considering that a cheap-o mic (like the kind that come with a PC) will do the job and only costs $5.
