Well, that's the funny thing about canonicity: something either is or it is not. Unfortunately, some fan doesn't have the say in what it is. Now, I hate those movies' stories (the visuals are pretty, at least)... but the only man that has final say of what is canon happens to be Lucas (the man that owns the property), and he thinks they're canon.

Do I think they these films screw with the series? Absolutely. do I prefer non-canon approaches to them? Yes. They're still canon.

Canonicity's not a value judgment as far as good or bad.