Hey all,
Ryan B. explains why it is necessary to have this Build Data checked into Source Control in the forum thread I have linked below:
The built data is the lighting information, and you need to save both the map and the built data when you rebuild. Otherwise the map does not know how to access the lighting information. This means anybody who syncs and loads your map will have no lighting information and thus the lighting will be automatically invalidated if they save the map.
Thanks,
H.