Well, some users like not to use a maximized window (eg, 2 windows side by side on a 1920x1080 monitor), are using page zoom or are viewing the website on a limited resolution tablet. Luckily the forum fits well on most cases, thanks to the ad being on the top. You can get the window width below 1024 pixels just fine, which is perfect for these cases.
BUT, when you have one of those miserable 1366x768 displays (even more infuriating on a desktop PC!), the height is SO LOW that the top banner does indeed ruin your experience. Sadly this is by far the most popular resolution on laptops.
If it isn't much work, I propose the following solution: have the ad relocate itself when the browser window gets narrow, otherwise use that empty space we got here:
Actually, the ad doesn't have to relocate. The content to the left and right of the ad could just shift to the bottom or the top if the windows gets narrow.