I've been doing a lot of browsing on my N800. It's screen is 800 pixels wide - and if the browser isn't in full-screen mode, it's more like 700 pixels wide. If the whole page won't fit in those 800 pixels, the overhang hangs off the right-hand side of the screen.
If something is going to fall off the side of the screen, I want it to be the ads and the navbar, not the content. Even if the browser manages to make both sidebars + the content fit on the screen, it the sidebars can take up a good 350-400 pixels by themselves. Dropping the left sidebar gives another 100-150 pixels to the content.
Of course, if you're reading this in an aggregator, the change will be completely invisible to you, and you'll never see the ads anyway. Meh!