If you have a backpack on your back, then how do you see a vault number or other details on your back?
Well - you're generally only wearing that Vaultsuit for the beginning portions of the game. I wouldn't want to be covering that up, either (since it's rather iconic - though it seems like you're not playing as a Vault Dweller in NV, so this might be a moot point, anyway...) But that could easily all take place before you get to your first shop and get enough caps saved up to buy a pack.
As far any other details on your back - I don't see how anything else was any more interesting to look at than a pack would be, when you get right down to it. (There was nothing so fascinating about the back of my Leather Armor that I couldn't possibly bear to see it covered, personally...)
For my own two cents, I think some gesture towards the concept that your character isn't actually managing to stuff all of this stuff into their pockets would be nice. I posted this over on the other "backpack" thread, but I always thought it would be neat to see a procedural system where where the more weight you carry, the more/larger packs appear on your character model. (Come to think of it, I almost seem to remember Dungeon Siege's mule doing that, though I could be wrong...)
I think the idea of a backpack, however, would work best as an optional accessory. Since I like to see game concepts incorporated (at least in some manner) into the gameplay, I think a minor weight capacity bonus for equipping one would be nice; but maybe not entirely necessary. But I wouldn't be against a variety of packs being available as mere aesthetic choices, either. That way it's a win/win for everyone - if you don't like the idea of packs then you don't need one, but it's there if you do.