I only own one lens for my camera (Canon EOS 60D), and I never take it off. Is there still any point in having the standard automatic sensor cleaning done on power on?
Yes. I just got back from a trip shooting at the Wildlife Refuges. I didn't remove the lens, and in fact, hadn't removed the lens for about the last 3,000 images. The last 300 images have a very distinct dust spot that appeared somewhere during the last day's shoot. Fortunately, in a place where I can crop it out without impacting the images.
Dust on a sensor is a generic term for any stuff that ends up on your sensor, and while opening up the camera and waving it around in a sandstorm is a great way to end up with spots, that stuff can come from anywhere, including things like microscopic bits of the inside of the camera breaking free or dust long hidden in the bowels of the camera sneaking out when you least expect it. And honestly, the last thing I expected was to get a dust spot on the last day of a photo trip like that...
Several models of zoom lens are designed in such a way that they likely inhale dust and may vent it into the body of the camera. How big a problem this is remains debatable (see this discussion for example).
Also, most lenses are not externally dust sealed so air and fine particles are likely to make their way in over time.
For these reasons it probably makes sense to use the sensor cleaning feature, at least occasionally.