i agree, you pretty much have to start this guy, unless he is a major defensive liability, relative to the team. and still then its questionable - im a fan of having a strong scoring guard or something playing backup to pick up most of the backup shots, and that can be very effective - so can playing backup at 2 spots. but if you dont already have decent scoring on the starting lineup, you pretty much need to reconcile that, first. the "play a star scorer" as backup, which ive recommended a bunch myself, makes sense when you already have sufficient scoring on your first line, and the marginal increase in offensive ability by having this low defense high offense guy start, is much less than the marginal increase in your backup line's offensive ability. then, that will clearly offset the loss of minutes for the player.
also, any time the guys offensive abilities make him a borderline starter or worse, the backup role makes sense, but that is effectively a statement that is always valid. the only interesting case to consider, to me, is when do you take a guy who is on his own, starter material (for your team, its all relative, of course), and play him at backup? to me the answer is never, when your whole team sucks and you need this one guy to bail you out.