Setting up a full-time mining rig is a tremendous resource commitment - and you either have to be all-in or go home if you want to ever actually see any return (yeah, I know - I'm ignoring syndicate/collective mining).
On the other hand, running a full node that doesn't mine is really not that expensive a thing to do - and provides all of the transaction validation, consensus voting, and block propagation that a mining node does.
For developers (and perhaps for users who just, well, use Ethereum a lot) there is a direct benefit to running such a node, but I wonder if there could be some way to provide incentive for other people to run these sorts of nodes. When I think about it I'm not seeing anything obvious and/or elegant.
I guess a corollary is whether or not it would even be a good idea. Seems to me the more nodes the better, though.