I'd imagine they are so ones has an easier way to copy a post # for use with that new ??code for post links (otherwise just mouse over the post link and look at the status bar). That said, I think just having it over on left side of the post itself would be enough. I'd also make it smaller and put it under the date. Having the numbers in the thread view breaks up the conversational flow when reading post subjects IMO.

Also I noticed it doesn't show on the post below this New Comment form.