I'm against it also, it's a pain if your trying to run a group buy and you need to always update the first post in order add the members and their names to the list.
Therefore if your selling something and you want to reduce the price or change the information or add pictures etc. you cant do it :S
And you don't have to open a new topic if you want to sell an item that is similar with your other items for sale in the same thread.
As a result by not opening a new thread SAU will save on storage costs because new threads won't be constantly posted.
I read that if you donate it solves the editing problem, but I have and still I can't edit :S