The issue of spelling mistakes is secondary actually. I'm sure nobody really minds spelling as long as the title gets understood. Though obviously good spelling in the thread titles will give a better impression of the forum to visitors.
However as others pointed out, it's handy in changing a FS thread to a SOLD one, for putting in "Issue Resolved" rather than having someone pick up the thread a week later and re-attempt to solve the OP's solved problem. This eliminates waste of time.
Also in certain cases (speaking from my experience here) often I get a pc symptom and googling leads me nowhere, so for example I post saying that I've got issue X. Two posts later I understand that it's all because of hardware Y. However I miss out having the experts of hardware Y to help me since my thread title is misleading.
I would like to know the reasons why 4 people voted against.
I haven't as yet seen a post saying the disadvantages which there might be for allowing the thread creator
only to change the title.
After all everyone can edit their own post even a week or two later, so why not the title?