These are my personal opinions about photo competitions:
I take part in photo competitions because it gives me (a non full-time photographer) opportunities to shoot with specific themes and purpose. It is through this process that I improve in my photography skills.
Sometimes I win, sometimes I don't. If I win, it is also a form of motivation and bonus in terms of prizes or cash to subsidise those expensive photographic equipments. Photography is a very expensive hobby!
If I don't win, its OK because photography is very subjective. It depends a lot on the judges. From photo competitions, I learned a lot by studying other people's winning shot. I don't see a point in keeping at home thousands of images I've taken.
In most competitions I've participated, they only take the copyright when I win a prize. And most of them will credit me if they will use my photos.
If you feel that the rules and regulations are not fair or the prize is not worth your photos, then don't participate!
Again Its just my personal opinions.