Trading Parker wouldn't be a horrible idea in my opinion, as long as we at least get a young PG in return, like a Monta Ellis or Raymond Felton kind of PG. Somebody with just as much upside as Parker (in their early 20's, etc), and someone we could plug into the starting lineup... Maurice Williams, Luke Ridnour, Delonte West, etc, etc. Lots of servicable PG's out there. None are "All-Stars" like Parker, but could likely mesh with the Spurs.
I don't know, I've never thought of our PG position to be an immensely integral part of a championship. At least not compared to the big man. Just has to be someone who can handle pressure, get more assists than TO's, and hit fairly big shots. Avery Johnson or Speedy Claxton; it doesn't have to be an All-Star... I look back and see the Heat topple the Mavericks with Jason Williams and an ancient Gary Payton running the point! Not exactly top 10 point guards...
If the Spurs are looking to improve through a trade (ie. getting back a package of young talent: PG + SF/PF, or, PG + C), Parker may be the best piece to work with. Plenty of teams would want Parker, which is exactly why the struggling Spurs should sell him as high as possible, while they still can. I'm sort of biased though - I'm really high on Ginobili and value him just as much as Parker (at least for the Spurs). Versitality is big nowadays, and Ginobili's skills and qualities are definitely varied... Don't get me wrong, Parker is definitely a steal, and I wouldn't blame anyone for thinking he's untouchable. But, sometimes you have to give some to get some, and for me, that would be our score-first-guard, Parker. I imagine for others, that'd be the aging Ginobili... but that's if we're talking trading one of the big 3 in the first place. Which I wouldn't say is necessary just yet.