From the second link:

>> Because Quake levels started empty, Quake had invisible "exterior" surfaces that required a separate process to detect and eliminate. If the level wasn't watertight, then the exterior could be reachable and the automated tools couldn't eliminate it. In contrast, because the Thief level started solid, this was never necessary. (I think Unreal's CSG may have started as solid as well.)

It's just unusual. I think C&C: Renegade should be 'solid by default' too.

One of the benefits is what you don't have a problem with the edge cases with an escaped ray casts and view points - you simply don't have casts and views into the infinity.