I registered a game once, got a lot of spam, and haven't ever since.
I also don't mail in rebates. Rebates aren't why I pick a product to purchase, so I don't care enough to bother...either that or I'm MsMoneyBags. Don't send in the registration cards for pretty much anything I buy, not just games - not only for privacy/spam potentials but also...I'm just lazy and forgetful.
It is a nice concept to be able to support the games by registering I suppose...but any giving out of my information uneccesarily isn't for me. There's enough of mine floating around out there as it is. There have been some feedback-only things I've filled out, and those are fine....but if a company wants general feedback, they don't need personal info to get it.