I'm not sure if either fryke or lurk is correct, but if seeing the hard disk light reassures me, what's the harm? This query arose out of problems I've always had with my iBook, the only Mac I've ever had. Certain programs will spin the beach ball for ages, and recently I've been using Wiretap to record streaming broadcasts. The most recent was an hour long, and then I got a beach ball for a long time after, and nothing else happened, although other programs ran OK, if perhaps slightly slowly. I wasn't sure if it was the computer busy turning a temporary file into a permanent one, or a crash that merited a 'force-quit'.
In the event the precious AIFF file eventually appeared after several minutes, but if I'd have seen the hard disk light I'd have been reassured and not been tempted to force-quit.
Have to say that "Basically, you'd be better off fixing the system (find out why those beach ball sessions happen...), since you then just don't need those LEDs" is not really an adequate answer. Having the hard disk light would *help* me find out why the beach ball is appearing!