Personally, I think this is a good idea. With SM coming out with f(x), it is important for a company to be competitive, and not to rest their eggs in just one basket.
In like manner, I don't think that comparison between Rainbow and Kara will be fatal, not unless there are very VERY serious differences (i.e maybe fashion, or stage projection, or character). I think in fact, just as much as Rainbow can learn a lot from Kara, despite being the Sunbaes, Kara can learn some things as well from Rainbow. As labelmates, they can help each other grow. Although at the end of the day, as in the previous posts, it's still "every man for himself" (or every girl-group for itself).
Rainbow has a lot of potential, though. Then again, I still feel that Kara's got more to show us, but in due time...
My two cents worth...