If you have extension tubes and the kit lens, I would appreciate knowing if they (what brand of tubes) fit together (whether or not you recommend that use). The question is why Canon felt it necessary to come out with a new model tube for this camera.
You said, " It's not like there's anything to hit, unless the lens would touch the sides of the tube." That is the question. Does the rear of the kit lens (probably the rubber bumper) touch the sides of Kenko tubes as it apparently does with the old Canon models?
Because of the new mount, two new extension tubes have been developed for Canon's EF lens range, the Extension Tube EF12II and EF25II. These new tubes replace the Extension Tubes EF12 and EF25 and work with almost every available Canon EF lens. Their new mount allows them to also work with the new EF-S 18-55mm f/3.5-5.6."
I agree that prime lenses are preferable to zooms on extentions (or for any use where changing lenses is not a problem). I would hope no one would use tubes with the lens set at 18mm. Even 55mm is really too short for decent working distance but might serve those of us who do not have the resources to buy the best quite yet.