I agree with Frogking, with no Soul in the body for about a month Ichigo's body would definately start to lose some muscle mass, and it'd probably need to be hooked up to a bunch of medical equipment just to keep him alive (for food and whatnot). Plus, logically speaking. wouldn't a body eventually die with no soul in it? It just seems more conveinient for everyone for Kon to be put in Ichigo's body.

Also: In that little thing that was at the end of the episodes for a while (that Dattebayo kept cutting off) where it was some kind of Soccer tournament or something, wasn't Kon in Ichigo's body then? I don't know whether this is filler though.