There is ALWAYS the possibility that there will be traffic on the runway whether there is a manned control tower or not.
I would speculate that there are more incidents at tower controlled airports than non-controlled airports with regards to runway incursions, near collisions, and actual collisions.
As a pilot that was operating from LAX in 1991 (
LAX Runway Collision ), I can attest that the control tower should be considered, at best, an advisory facility with only the power of suggestion over your aircrafts movements.
The ultimate authority for decisions comes from the cockpit. I think the DCA pilots did a fine job. Their decision can be second guessed to death (and is being second guessed to death), but they had safe, uneventful landings without an operating control tower - which as it turns out happens with part 121 carriers hundreds of times every day.