I havn't explored this yet but from what I've seen there is no competition or extra bonus for modeling a building. I think it could be really successful if it was turned into some sort of game.
See the works of Luis von Ahn (http://www.cs.cmu.edu/~biglou/), specifically his GWAP project (http://www.gwap.com/gwap/). He has created several web games that provide training for machine learning algorithms while at the same time entertaining users and brining them back for more. The games are very successful, and from the talk of his I heard, it sounds like the data is proving useful.
There are a lot of people who really enjoy this sort of thing. For example, there are online forums for skyscraper enthusiasts that attract pretty big audiences. I think given the interest out there, it will be successful regardless, for much the same reasons that some people enjoy woodworking as a hobby.
Some way to publicize one's work -- to get more recognition for it -- would certainly make it even more successful though.
The models you create will be in their 3d warehouse and are keyed to your Google account. So if you're an awesome modeler, unlike me, and/or you live somewhere people want to look at buildings, then your model will rack up downloads among sketchup users.
Incidentally, to make this work the coverage areas seem to have multiple aerial views, not unlike the maps on MS Live (or Bing maps or whatever it is now). Seems as if Google has quietly taken up doing streetview-air.
If so, that's a really great idea on how to get high-quality work done for lots and lots of cities, for almost free...