Chewie in ROTS didn't bother me, I think it was not too much, and was "believable". After all, the battle takes place on Kashyyk and Chewie is a pretty good wookie warrior.
But Han would surely have been TOO much, and it goes against the EU, especially the "Han Solo Trilogy" novels in which we see him from his childhood to ANH... And I just love the way that this "smuggler" enters in the OT story, he's a totally "exterior" character, coming in just by coincidence (or by the Force, if you prefer...).

I'm really really glad that we don't see Han in ROTS, thanks!