Sure, which is why those guys are usually available at a discount in both dynasty and redraft.
The question should be, how steep a discount should it be? In redraft, you'll probably wait a game or two before feeling safe starting lacy (or rawls) unless you went zero rb and he's your only choice. So you have to discount him by the games you won't use him and the odds he won't be good enough to start (including not being good enough to take a lead role but still being in a committee) and his near zero exit value because of he doesn't start, it's probably because he's too fat. That means you drop him down a lot. But the upside of him becoming the next lynch is huge, which is why these kinds of guys tend to go in the 6th/7th/8th of redraft.
In dynasty, you drop him down for different reasons. It's not nearly as big a deal that he might be in a committee, because the upside of 4 years of being a starter is so much higher than the upside of 14 games you'd start him in redraft. There's a risk that he will have zero exit value, that he will be mired in a committee, that he will not play at all, etc. Those are all real risks. And you're stuck holding them while you wait so there go two roster spots at the start of the season which is when they're most critical for picking up the surprise waiver studs.
But the upside if one of them does become the rb1 is potentially 4 years of rb1 numbers in a good offense with a stable coach, qb and defense. Thats potentially second or third round startup adp if they hit.
So the question isn't, are there more cases of committee situations sucking than of them being good, but how much more likely does failure have to be than success for you to not invest a second round pick and two roster spots. It's like a pot odds calculation in poker - in the long run if you do this a bunch of times you'll probably come out ahead.
The only time I wouldn't take a gamble like this is if I had a deep roster and shallow roster size and just couldn't consolidate it any more. But I am constantly looking for ways to add these speculative plays that add immediate depth and some chance at studliness. These are the kind of plays I make room for.
it would be a mistake to assume the Seahawks outscore the #2 offense by 15% this year. They might be a top 10 offense but nothing about this situation or backs should lead someone to believe this becomes foreman / Coleman. 
