it depends on your league's format, mainly bench size and if there are position restrictions... with a bigger bench I could see handcuffing more backs than normal..
generally speaking though, at least for redrafts, I wouldn't handcuff more than a couple backs... let's say, you start 2 RBs and only have 2 bench spots for additional RBs... I wouldn't use one of those backup spots for a handcuff unless it's a larry johnson caliber backup or the handcuff is the best rb available at the time of your pick... you'll naturally need at least one bye week RB and I'd rather take a RB with upside for my other back than most handcuffs... a lot of RBs available around the time of handcuffs in smaller bench size leagues are RBs in RBBC situations who have more upside than a handcuff... I'd also rather have a backup like chris perry than a handcuff for most other RBs even if I don't have rudi
In my opinion, it makes much more sense to handcuff one of your players rather than to draft the backup of a player you don't have on your roster. I say this because if your starter gets hurt, but the other guy's starter doesn't, then you have two unusable roster spos instead of one. While it's true that your backup, say Michael Turner, isn't nearly as good as Larry Johnson, he's still better with LT hurt than Larry Johnson with a healthy Priest...well maybe not in that case, but you get the point I'm trying to make.
yeah, i dont think you should draft a backup for someone else starter, unless you do it with the intention of trying to trade him to the team that has the starter.
The only exception, is if the starter is somewhat injury prone and you beleive the backup to be of stud caliber.
Example would be last year, everyone was grabbing up LJ in the presumption that holmes would eventually get hurt, knowing full well that if that happened, LJ would be a top back in the NFL.
I cant say there is really a situation like that this year, except maybe drafting some young guys who are playing behind old guys, and even then only if you happen to be in a dynasty league.