Working under the assumption that he comes back healthy, it comes down to the one thing that sets him apart.
Bradley is an all-NBA defender who is young as heck and just beginning to show an offensive game that, rough as it is, can put in 12 a night, including a few for the highlight reel. His defense can give a whole team an identity, though. He can take a game over in crucial minutes with stops and defensive intensity.
The other guards we have are literally a collection of 6th man candidates: former 6POY winners in JET and Barbosa, plus Courtney Lee, who seems like a nice big guard that could look really good off the bench. None of those guys are all-NBA anything, though.
So how important is Bradley? It could be the difference between being good, middle-of-the-playoff-pack 2nd round regular hoping things come together (basically, that two 6POY = 1 Allstar at the right time), and being an elite team and perhaps contender.
I think there's a reason the other three SG's have had their greatest success as 6th men. Terry's undersizedness and questionable D are masked, and his offensive talent is maximized. Barbosa's aggressiveness and experience shine, and he can cover for his size. Lee is just more of a journeyman.
Bradley is elite at something, and that sets him apart. I think we have a stronger case to be a contender with what Bradley brings than any other guard, save Rondo.
With a healthy Rondo and Bradley in the playoffs, we could probably get away with only one of the three other guards, and it probably doesn't matter that much which one it is.
Bradley is pretty important, IMO. I think he's also first to get traded, so go figure.