For one thing, backup QBs and backup RBs are completely different issues. In average leagues (non-keeper), I definitely agree that backing up your QB generally isn't a good idea (although there are exceptions, such as when you're using a QB-WR combo strategy).
With RBs, on the other hand, it's not a matter of whether to draft backups, but, as Guttpuppy and hayes mentioned, whether to back up your own players or to grab another team's backups. Assuming all other factors are equal (which they usually aren't

), I think I'd go with the RB backing up another team's starter rather than my own more often than not, but it's a very tough question.
Do the majority of backup RBs wind up being useless? Yes, but so does every other player you'd pick at that point of the draft...
Sure, you can add, say, a second bench WR to your roster, who will definitely put up stats, unlike the backup RB. But what are the odds that WR will wind up helping your team? And even if he does, the improvement over your other WRs will most likely be minimal. If that backup RB pans out, your reward will be much greater, potentially enough to tilt the balance in your league.
Last year, Priest Holmes might have been the clear-cut fantasy MVP, but it was Clinton Portis, a late-round flyer who looked like a backup (at least in relatively early drafts), who wound up being the difference-maker in most leagues. Even in 1999, when Kurt Warner wowed the fantasy world, Olandis Gary wound up being the key to many fantasy titles...