If I land on that particular dialogue, it's because I've messed up the
message I was writing really badly. I just discard it by default. That's
a different dialogue than say when Thunderbird is having issues saving
to the sent folder for some reason.
Usually I get the dialogue that it's trying to save a message, but
can't because of network access errors. In that case I knew it was going
to slow everything down, so I wouldn't save the message either.