I have to vote other. There are several reasons.
1.) I use Linux at times and WMP, WinAmp and RealPlayer do not work on Linux. At least i dont bother to install them. So there i use Amarok and Mplayer.
2.) In Windows i use WMP, Zune, WinAmp, and VLC. WMP for mostly video's and avi files. Zune for some music. WinAmp is my main music since it can play my while collection while there are files that WMP and Zune can not play. VLC for those stubborn files that dont play on Windows apps.
3.) Real player is a hog. Just install the REal Player Alternative codec along with the Quicktime Alternative so you dont have to isntall either app. They stink.