Why I spend hours by sitting in front of WME and editing?
To have Waze trusthworthy. (New) users are evaluating the "product" and comparing it to other (commercial) products (like Garmin, TomTom, Navigon, Sygic, etc.). If they find product unreliable they will quit easily, and map quality is one of most important criteria, together with quality of routing algorithm for sure. so high quality maps allows community to grow quickly and everyone would benefit from growing community (more data for reports, jams, average speeds, etc.) and also bringing more new editors on board.
I countries like ours, without basemap, so it was just blank space here, after short time first enthusiastic editors were able to put main road network and consequently improve map. so the product was ready to be used, but far away from quality of competitors, but with online traffic information. at this time many new users were confused by insufficient quality of map and left... by time passing more and more editors get involved and map quality was improving (and is still)... and resulted into community growing faster and faster.

