I think it's just a matter of options. Walker is a 3, and Giddens is a 2. We don't have any real 3's behind Paul Pierce, so Doc is almost forced to play Walker. We have a number of players that can fill the 2 spot behind Ray: Eddie, Stephon and even Pruitt are ahead of Giddens on the list right now. I'm sure if we had more options at the 3, Billy wouldn't be playing either - in fact, when TA and Scal were healthy, he didn't.
For what it's worth, though, Danny still seems to believe that Giddens has the skills right now to contribute to the team if given the opportunity. If nothing else, that bodes well for next year.