@Derek Morr 
What is your definition of "useful"? It is the subject sense as the term "useful" have multiple definition and every person have different sense of how the online information is useful or not. Your feel of not useful is not mean it is objective not useful.

To determine what useful or not, please think the core question:Why we need have the such thing as map? Map is the tool to provide the accurate geographic information, include driveway, in the sense, mapping driveways or anything exist in ground is useful:provide the real geographic information

This thought answer the question of "why you think adding driveways to unmapped houses is useful"? 

It's simple:It is the real geographic information, represent the existence in reality, if mapping trees, ponds, swimming pools, walls, or other objects existed you may feel not useful, it is still useful by providing the accurate geographic information represent the reality, this core thought also meet "on the ground" principles of OSM

Regarding on your another question:driveway that intersect on house you added and gave you the validation error, the solution also simple:Either dragging the nodes of driveway out of building you added, or just ignore such warning if you feel too much burden to you as the driveway not far exceed into building, such "validation error" won't prevent you from upload your work of adding houses that intersect with driveways, and I can help fix the intersect between driveways and houses.(I mapped them according to the aerial imagerys, so where driveway terminates I can clearly know)