Having lightmaps stored as part of the .umap file is fairly problematic for distributed teams (at least those without redundant OC-3 lines coming into their building). Any change that affects lighting causes the entire map file to be marked as a diff. This means hundreds of megabytes of data being pushed up and pulled down if even a single item in the level that affects lighting is changed.
Moving the lightmaps to some kind of storage external to the level file would be very helpful for distributed teams and much kinder to everybody’s source control repositories.