I agree man, i drove me crazy having the T-Mobile sync account! What it is for is your Mobile Backup (auto built into this phone unlike the last one). I have all my contacts on Google and link them to the people's Facebook, and that's what I like!
However, I noticed how obnoxious it was to have duplicate numbers for people who have a Google contact & a mobile backup contact. Also if you use the "Faves" function, it will auto create a t-mobile-based contact to sync with the website. So when you log in, you will click on "Faves" and see all of them there, in addition to Mobile backup.
I got pissed off, deleted everyone off mobile backup period, and backwards synced to my phone so there are NO mobile backup contacts. I said screw the "Faves" feature too because of how annoying it was. I really don't feel like I'm missing out seeing as Faves is just an upgrade to the myFaves UI for Android phones