Perhaps it would be more aesthetic as well as more efficient if they stuck the ad between every 10 posts.  That way the forum remains centered and easy to look at, but readers are forced to look at the ad as they scroll past it.
If the ad is on the left, readers won't be able to see it after a few posts - plus, the cursor generally rests right of the screen, so people would have to put effort into moving the mouse to the left to click on an ad.  Some basic GUInterface guidelines is that the top and right of the screen is the easiest accessible spots, hence the in-efficiency of Microsoft's start menu...