I can see that it isn't an obvious choice if you aren't exclusive, but I'm very glad I've participated in assignments. My shots are exclusive anyways, and they do much better than my average photo. I made a quick lightbox of my assignment photos, which allows me to see the total revenue. My assignments have brought in almost 10% of my total revenue to date, and represent less than 1% of my portfolio. My most recent entries this month already have a download:
With the new pricing structure, even a subscription download of assignment files nets you 3 sub credits. So I recommend considering submitting for assignments for the extra exposure. The 100 download 'credit' ranks you highly in a search based on downloads, and any credit sales are substantial in value. Of course, you stand a chance of winning as well!
How have you done with your assignment entries?