Yes. And if the DEMS defeat him, well, we could get a Lieberman-esque Dem who would, in many circumstances, be a better vote for us than him. I am NOT endorsing voting for a Dem, just pointing out reality.
Your point is certainly not without merit. One must also keep in mind, however, that if the GOP loses bunch of Senate seats this fall, just having Chaffee in the "R" column could keep us in the majority. I don't think that will happen, but you never know.