We need to see what Sage can do over an extended period of time -- at least 4-5 games. The fact that he's been a backup for several years doesn't mean he can't be a quality QB. It's been done several times before (Trent Green and Jake Delhomme are current examples). Sage has played well in the preseason and he has pretty good physical skills.
Lets say Sage starts for 6 games. He doesn't light it up, but doesn't suck either. Lets say 58% completions, 220 yards per game, 10 TDs and 6 ints, with the Fins going 3-3. To me that's a sign that he can be a quality QB, as those are pretty decent numbers for a 26 year old guy's 2nd-7th pro starts. In our offense, with Gus's perfromance as the basis for comparison, even 55% for 185 ypg, 7 TDs and 6 ints is respectable for his age and relative inexperience. It would enable us to start him next year (and probably draft a QB). If Sage plays well, we'd be in a situation like SD has now, with 2 quality (or perceived quality) young QBs. If not, at least we wouldn't be wasting a roster spot on a 35-year old journeyman who plays no role in the future of the team.