For some reason, when you hold the mouse button down in OS X Panther (and Jaguar), the processor usage goes up to 100%. It doesn’t matter if you’re clicking nothing on the desktop or the most complex of widgets, holding down the mouse takes up all your processor power.
So what? Well, here’s one example of when it might affect you. If you’re listening to online radio (or an mp3 in iTunes) and reading a website or document that requires scrolling and you decide to hold the mouse button down on your scroll bar, often your internet radio will cut out or your mp3 will skip. Especially using Safari for whatever reason. I’m sure there are other instances where this could be problematic, especially when rendering video or 3d.
I’m on a 1.6gh G5, so it’s not that I’m using antiquated hardware. This is, as far as I can tell, a fault in OS X.