The thing is that this "feature" would probably be more used for those who want to keep playing a map after failing (without NF), but not to start a short game to wait for other people to come.
The ability to keep playing after failing without NF is really neat for a chunk of players that may not like the lack of pressure with NF, or maybe its their first time trying a map, and they want to make sure that they finish the map, but it may be possible that they FC the map first try so they don't really want to have NF on which reduces the pp gain (happened to me before).
The implication would be that if other players know this, they would probably open a locked match room just for the benefit mentioned above. The point is, it can be the that more and more people would do this, and we CAN be flooded by thousands of solo rooms. Not many players know about this now, but still. This is a roundabout way to get around the whole failing-the-song thing.
The better way would be to have an option to keep playing after failing, but we all know how well suggestions are in osu development in recent days.