This actually holds true more for UNIX in general than computers in general. UNIX performs certain cleanup activities late at night when nobody's on, and when one constantly shuts down and restarts the computer, avoiding this cleanup time, stuff starts to build up (hence the reason for Macjanitor). But that still doesn't mean Powerbooks shouldn't have an easy way to force shut down the computer without pulling the battery, in case everything, including the keyboard, locks up.
Actually, if you hold the power button down for several seconds, it should shut down. I just remembered about this because that's what the kernel panic box tells you to do.