I think BJJ is better with out the structured grading, just because some one can remember a series of movements doesn't mean they can do them in a live situation.
Same as if a white belt triangles a Purple, doesn't instantly warrant him getting his blue, the instructor will be keeping a eye on him and will probably roll with him to see if he is ready.
It seems more organic that way IMO.
Also I think the best way to go out and really see if you're worth your next belt (if you really care) is to compete against people better than you that way even if you're not going up the belt ranks you're still proving you're on the same skill level as some one with a higher belt rank than you.
But they're only for holding your pants up any way.