I'm a contributor to OpenStreetMap, and last year I helped Code for America import thousands of buildings.

OSM is rightfully skeptical of uploads. Even when cities give specific permission to OSM, it's up to the community whether a large import is helpful or risks drowning out local mappers. If anything, this site would be helpful to download a subset of the city's buildings as KML and use JOSM to upload.

This site exists as a custom mapmaker. It's easier for a non-GIS user to get data out and make custom maps compared to OSM APIs and tiles.

It's interesting that you mention custom mapmaking. Google provides a similar service - creating a KML file hosted from a Google Spreadsheet. Here is the newest version: http://www.google.com/earth/outreach/tutorials/spreadsheet.h...

I've recommended it to a non-technical family member (he's a realtor) and they were able to create and host their own kml-based maps within an hour's time. Note that it does not use polygon based building markers, but rather addresses (and coordinates), which are simpler for non-GIS people to use.

I actually built something similar idea wise just because it seemed like a fun project to play with (http://roofmapper.com). However my implementation was very simplistic in comparison.

That's cool, although I wouldn't rely on the Google map draw tools. It's cool to learn and have personal projects, but advertising yet another data collection tool without a clear idea for where the data is going or if it's even usable seems counter-productive to me, since we have a project like OSM.