Interestingly, one of my least-favorite parts about "doing well" at hackathons like these is that I don't get the opportunity to go around to other tables and see what everyone has else made, and talk with them about it. Sometimes, really cool projects and people go under-noticed (I've been there as well).
But I have to admit that sometimes there are pretty nice prices - like on the last gamejam I attended there was an organized trip to bigger gamejam for the winners (except of small stuff like T-shirts). There were no judges, all attendees voted on their favorites. That's in my opinion the true hackaton/gamejam spirit.
I would attend a university sponsored hackathon that had little / no prizes. I would not attend a corporate sponsored one with bad prizes.
For example, AT&T recently held a hackathon in Atlanta. The prizes were meager, and it was clear to me that ulterior motives were present. Why would anyone attend a hackathon like that is beyond me?
A lot of corporate ones are set up with contracts that leave your work at their mercy to continue without you. Unless there's a prize - it isn't worth it with these types of arrangements.
For social good, civil, city government, university etc - I'll do it with no expectations just to make a difference.
(We did award SXSW 2015 badges to the "audience favorite" but that was a surprise at the tail end of the demo time.)