I guess the problem would exist for someone who is new, who just wants to delete a post after a reply has been received.
I've seen it happen, even for the edit function, in another forum. People spend a lot of effort to help someone, and in the process, leave something of value to people who come later with similar problems.
The person who asked the problem is happy with the answer, so they delete the contents of their original post. The whole thread becomes pointless.
In this situation, the mods can fish into email notifications and restore the post, but it's a bit of a pain.
The thread delete might be better disabled (except by a mod), if this (or the next) version of the software allowed it.