I think it's less about being dumb, but more just holding onto a sliver of hope they get called up to the NBA (which I guess could be considered dumb). Gaines goes to Europe, he isn't playing in the NBA right now. That is true for most D-Leaguers who get called up. Some stick, some don't, but at least they got to fulfill their dream for that 10 day contract.
I suppose, but they're just not playing real basketball and getting paid crap. It doesn't seem terribly wise to do anything but do your best to become your best, and the D-League doesn't seem like a great place to experience the growth that teams are looking for. Teams want WINNERS! (Said in my best Mike Singletary voice). Actually seeing the world seems pretty cool/important, too.
But that's my opinion. I just don't think most people, including talent-evaluators care much for the D-League itself for finding people since it's basketball not rooted in competitive team basketball. But you make a good point.