It might be the fanboy in me, but I truly believe the jury is still out on Smith. The dude's been through a lot so far in his NFL career, from jumping into one of the worst teams I've ever seen assembled, to battling with injuries, to back-and-forth decisions at the position and no continuity at QB, to a different offensive coordinator and a new gameplan every single season. I think Singletary feels the same way, and wants to see how Smith progresses under center next year as he'll be under the same offensive gameplan for the second straight season, and has been "guaranteed" the starting job this offseason.
With that being said, I could also see the 49ers making a move for a veteran QB...if you have a chance to upgrade, you upgrade.
I also want to point out that McNabb is turning 35 this year, and Smith is turning 26. How much does McNabb have left? Can Smith grow into a solid option under center in the next 2-3 years? The clock is ticking on McNabb, and I'd rather not throw away a first round pick that could land us a 10-15 year player and our current starter for a guy who might have five years left. I'll agree that McNabb is an immediate upgrade at the position, but does it catapault us into Super Bowl contenders? Playoffs, yes...but top of the NFL heap, I don't think so.
I would be ok with a 3rd round selection and Smith for McNabb, but I think the true value would be a 4th or 5th rounder and Smith for McNabb, especially considering the trades I've seen thus far this offseason. I think teams are starting to value draft picks more than they have in years past.