IMO, all four teams have significant question marks for at least 1 of their RBs.
Dolphins: One point where I seem to disagree with a lot of people on this thread is that I'm not quite as sold on Williams as a lot of you are. He had a solid year as a backup last season, certainly, but the backup RBs are good on all four teams. If either Williams or Brown goes down, which is a very distinct possibility given both of their recent injury histories, do they have the depth to cope? Not sure that they do.
Jets: Thomas Jones will turn 31 before the season, is mad about his contract, and has logged 290+ carries in each of the past four season. Since his workload was pretty light over the first four years of his career, he still doesn't have a ton of mileage on his body, but the Jets absolutely need him to stay healthy. Leon Washington isn't your standard feature back ( little guy, <150 carries over the past two years combined), and I'd be surprised if they tried to make him one even if Jones did have a down year.
Bills: Lynch is a headcase, admittedly, but he's also an awfully good young RB. And Jackson proved himself to be an excellent #2 last year, who was capable of stepping up and playing as the #1. With Rhodes as added depth, they can absolutely survive the loss of any of their RB with surprisingly little dropoff.
Pats: Same as the Bills, really. Maroney is an injury question-mark, and Morris and Taylor are both age question-marks, but they have easily the best 3rd down RB of the group and can survive the loss of any one of those guys just fine.
Basically, in this day and age it seems like you pretty much have to plan for injuries. Going 3 deep at RB is essentially a requirement, especially when your primary RB (or RBs) is aging or injury-prone. The Bills and Pats have depth and the Dolphins and Jets, comparatively, don't.