This would only be a good feature if I could turn it off
I don't want anyone being able to snoop around in my Zune to see what I like so they can target advertising to me (and that's what promo tracks are - a form of advertising). If it's anything like Netflix or Amazon's recommendations - based on what I've already bought and/or have on my wishlist - it will be more of an annoyance than a beneficial feature (if I were to allow it to do it in the first place, which I wouldn't).
Next thing you know, the RIAA will be suing you because some music service's snoop-bot discovered tracks on your Zune they couldn't verify as legally-bought.
That might be my paranoia speaking, but I wouldn't put anything past the suit-happy RIAA at this point.