IMHO Vince will get better at the passing-game during the preseason and you'll come to see him more as a QB who can improvise/run, instead of a running QB.
If Fitz got hurt and you put VY at QB and gave him 8 designed running plays a game, from the wildcat or not, he's going to get hurt before you know it, then you're down to #3. Same with Brad Smith. Neither of those guys are built for that kind of beating in the NFL. In fact the 2nd team 'running" QB would get extra attention from the defense once he crossed the LOS, and Smith and Young are not compactly built like Tebow.
At the same time, if Fitz was healthy you could put in either of those two guys for a change of pace or to stick it in the endzone from inside the 7 yrd line. I know Vince is very good at that.
So...if Fitz goes down, the 2nd string QB needs to be able to distribute, which means he must be at least a decent hunker of that thing we call a pigskin.