For me, the biggest thing keeping me in the MP3 realm right now is utility.

I have an entire suite of useful tools that work only with MP3 files and can't be used on WMA files. Things like MP3 Tag Studio, GapKiller, etc. And I can rest comfortably knowing that I'll be able to transfer those files to any type of player or stream them to any networked player I buy in the future. I don't have that with WMA.

If WMA can pull off better encoding at lower bit rates, that's fine. But since I've got an empeg, disk space is not my limiting factor any more. Once any encoded file format is up to the level where I can't distinguish it from the CD (as with properly-encoded high bitrate MP3s), then all I'm left with is the other features of the file format as the deciding factors.

I do occasionally encode things in WMA, such as voice recordings at low bit rates, because I understand that it can do better in those situations. But primarily I stick with MP3 because of convenience.