I think this option is small but very important.
When I close my board, I would like to have the choice of allowing the users to read the forums, but not be able to post. My logic is explained, below.
Closing down the board completely is needed sometimes, so keep that option.
There are other times we may need to make some changes, and having the board read-only during that time would be perfectly fine. It would be like a single global permission to prevent additions or edits to everything, including threads/posts, user profiles, etc.
FWIW, during upgrades, a few boards I know will close the forum briefly, make a copy of their database, and then use that copy to reopen the board in read-only mode (by changing usergroup permissions) while they perform the upgrade on the other database copy...then switch to the new directory and database when it's ready. I'm considering that when I upgrade one of ours in the near future.