I have no significant knowledge of how TOR works, so I might be off the mark here. Perhaps one incentive is that by running your own node, you can utilize it as an entry or exit node for your own activities over TOR. By controlling either the entry or the exit node, you know that a bad actor does not control both of the nodes involved in your own usage. Just a thought. Maybe this strategy is flawed somehow. Please chime in and correct me if you see a flaw in this strategy.