Is it because the frame rate is still relatively slow compared to the relevant time scale for the human eye? What if we had the option of a million fps? Would we still need motion blur?
More importantly, is it OK to not use it? I made a nice lower third thingie that has two adjacent parts. But when I use motion blur, the parts come apart and then I can see the layer underneath which was not supposed to happen since they were placed adjacent.
The shutter speed of a camera is usually low enough to capture motion blur, but creating motion graphics doesn't involve a shutter of course, so each frame is perfectly sharp. Standard frame rates are too low for human eyes to naturally blur the created motion, so AE provides a way to simulate that. I believe around 200 fps is the point where the eye doesn't need fake motion blur.
I say it is perfectly ok not to use motion blur. If it looks better with it off, go with it off! A lower third sliding onto a screen doesn't have to simulate the real world, since it's not in the real world. It all depends on the look you are going for.