Well, keep in mind that Slott brought Kaine back. I think he did so based on the idea that it was a limited run thing that he could let another writer play with... and then has other plans that Kaine will get folded back into.

I think Kaine is a big part of the meta-arc though and that we may well see Kaine with Peter's mind or even as a vehicle that Ben Reilly's soul winds up attached to by the end.

It helps that Kaine is a tragic figure. I think Slott had a tragic end to Kaine's story in mind the minute he brought him back and that his use so far is largely to set him up for a preordained role later in the story.