If this mechanic were removed, you would have a lot of cases where people do a portion of the dungeon then mostly leave but 1 person stays to hold the instance open. That way can begin again later in the middle. I know because I've done this in other games for days on end.

Technical issues arising from that already discussed.

There was some Dev discussion of a "minimum item level" to be implemented. People too severely undergeared shouldn't be there at all. Go do something else to gear better, then come back.