That seems kind of...pointless. All it would provide would be aesthetics to a build possibly, but lack any real functionality. Classic's commands are very functional in their design and usage and only help the user with building or patrolling for grief and the like. Maybe I'm being too curmudgeonly about the idea of a /door command, but I don't think so. I'm thinking about how cool+ users can use the /water and /lava commands for their builds, and maybe /door could be implemented the same way, but I'm not even sure doors are an item block available in the Classic game.
Not really. It would probably take several hundred users all using a /door block at the same time for there to be noticeable lag on the server. When someone is banned and their work is reverted, it takes several thousand blocks being reverted for there to be any noticeable lag, and it's only for a second or two.