I think the other thing that's somewhat gotten lost in the RB conversation is that Portis had what easily could have been a career-ending concussion to end his 2009 season. He has yet to play a down since then. While we expect to see him this year, there is absolutely no guarantee he doesn't take a hard shot (because he's GOING to take hard shots) and be done, either for the season or forever. It could happen in pre-season given his history.
That happens, we're going to be thrilled with these moves, and while part of me sees Shanahan trying to light a major fire under his starter's ass here, the other part remembers these RB additions may have other concerns driving them - #1, Portis' health.