Interesting. I see your point with first alternate, but I am unsure why it is a bad thing for 1st alternates to be able to withdraw easily.
For Roddick, I hope he'll stay in the rankings as he's got a nice series of weeks ranked in the top-50 and top-100. No reason not to prolong them IMO.
As for Sampras, I always thought he was unsure whether or not he'd continue playing. He suspected 2002 USO was it, but if in doubt then you shouldn't really announce retirement, should you?
It's statements like that that make me go, really, you can't be serious, I say be done with him and don't let his ghost linger, things like this make me wonder about tennis fans sometimes, there is no good argument that can be made for having a retired player still in the rankings.