For what it's worth, the slow importing in iTunes due to sound level checking can be disabled. If you do disable it, imports are very fast.
A big advantage here is the native support in iTunes for good and proper ID3v2 image tags. Winamp's support for them is nonexistant. WMP's support for them sucks, and neither give you an api to fix it. The API's that almost all of the frontends are using do not support embedded album artowrk either. Why? You've got me! Tagging your music properly is just a no brainer. Every piece of music I buy or rip gets tagged properly, and iTunes happens to be adept at handling and organizing music by any tag (It also happens to be a superb tag editor, especially on the mac platform where you can write applescripts to assist your taggings). Anyway, if nothing else, using iTunes instead of Winamp as your frontend's slave-music-player of choice will get you a lot of things for free that winamp's limited API will not.
FWIW, on the mac side there has always been an open sdk for iTunes (and indeed every single application Apple produces) via the embedded applescript dictionaries. I really see a lot of good things coming from the mac folks on this forum - I think it will suprise the community when it gets done