I think regardless of what uke does it is an opportunity for nage to train. To me a good uke is one that doesn't act like we expect. Even if their ukemi is not good, it is then up to nage to protect them. If they roll away before we "do anything", then why is that? Did nage not invite them in and make uke feel welcome? Many times I notice that nage just wants to do "something", without regard for uke. If uke does not give you the energy you want then nage must deal with that. There are so many people in the dojo with different opportunities to train on many different levels why must everything be about the "throw or lock". I thought it was about harmonizing. Not every technique will look "classical".
Now with all that said - if uke is just being an a&%hole....