When I create a thread prefix for say "Done", I wouldn't mind to select from a few selections: delete thread, or close thread, or whatever else might be logical.
So when I am browsing the forum and it's used for support tickets, or sales questions, that the support guy could change the thread prefix from [active] to [solved] and the thread would also be closed.
I could ask the same about thread prefixes,
can't the mod not just add [plaintext] prefix to the thread? ..
Hence why it's a small tweak.
On bigger sites where every second three new questions are asked, this cuts the time in half and saves time and money.
It's a simple request, the devs are free to ignore it.
Or, have a php callback support, so we can make a plugin for it, that way we can also extend it when the it's marked as 'Done' that a canned response is added at the bottom of the ticket.