The assignments are a pain in the neck. The chance of getting an image accepted as an assignment is dismal. It is alot of work!
So why participate? I participate because it pushes me to think a little harder about my shots. I participate because it takes me out of my comfort zone. I also participate because even if an image is not accepted for an assignment, I get some new and different material for my portfolio. More times than not, images that I have rejected for assignments are later accepted when I submit them as regular images.
I think we as photographers sometimes get caught in our own trap and tend to shoot the same way time after time and we tend to shoot the same subject matter. An assignment is really opportunity to reach out and try something new!
So when the next assignment comes ... try not to say "This assignment looks to hard", or "I don't like the subject matter". Look at it as an opportunity to try something new, try something different, get out of your comfort zone. Think about the concept and who knows, you just may get some great stock photo shots!