I guess I see it as a duty to some extent, plus I get a kick from contributing code. But honestly, the main reason is business. We want other companies to use our mapping platform, which uses route-me. When you get introduced to someone as a "route-me expert," it makes closing the deal about 1000X easier. Companies like Red Hat and CloudMade were built on that principle.
I also think the death of open source is greatly exaggerated. It's natural that contributions would fall off, as FOSS gets more mature, and just requires polish. As it turns out, it's not nearly as fun to polish software as it is to write something new. And there's endless polishing work on all software.
This may be the single reason why open source isn't more dominant - because you actually have to pay guys to do the dirty work at the end, or they'll go back to hacking something more fun.
We see it as good business. We are known to be route-me experts, which has helped us sell our platform for mobile map dev (and dev devices).