One thing I think is being overlooked in the McNabb-Vick-Young-Grossman/Beck comparison is the effect of the environment on QB performance.
Andy Reid is the example I'm going to use.
Andy Reid, IMO, has a knack, or skill, for getting the maximum performance out of a QB that may be among the best, if not the best, of any HC in the league. One reason for this, again IMO, is his flexibility in adapting his system to maximize the upside skills of a QB while simultaneously minimizing the downside deficiencies of that same QB. We probably saw with the Eagles the best Donovan McNabb that could be gotten and are seeing the same thing with Michael Vick. Not every HC has that ability-in fact I think few do.
The problem I have with bringing Young here has more to do with Mike Shanahan than it does with Young. Now there are a lot of things I really like about Shanahan but there are also some things I don't particularly like. One of these is his inflexibility. "My way or the highway" has an appeal in terms of establishing a disciplined environment but it also has limitations when dealing with talented but problematical players. We've already seen two examples, one of his own making (McNabb), the other he inhierited (Haynesworth). Neither of these was entirely Shanahan's fault but his inflexibility most likely closed him off from what might have been workable solutions-his approach seemed of the "pounding a square peg into a round hole" variety.
Jeff Fischer lost his job because of his inability to successfully incorporate Vince Young into the system in a consistently productive manner. Again, it wasn't all his fault but as HC he has to bear the result. Precisely why he wasn't able to do so I don't know. Young is definitely a "problem child" type of QB, but so is Vick although his problems aren't of the same variety. I would venture that if somehow Young were to wind up on the Eagles roster you would wind up seeing the best performing version of Vince Young that could be gotten-quite possible better than on any of the other 31 NFL teams.
This is why I question thew wisdom of bringing Young here. Shanahan doesn't, IMO, have the "knack" or flexibility to be able to successfully deal with and thus maximize the performance of a player that isn't already easily moldable, or coachable nor does his philosophy incorporate enough flexibility to "tweak" his system adequately to accomodate players like Young-or Vick-and bring out their maximum capabilities.