A B737-300 flight crew failed to make a descent crossing restriction and the First Officer cited the consistently poor vertical navigation performance of the FMS.

Narrative

On STAR to FLL; cleared to cross FATHR intersection at FL240. The VNAV in this series of aircraft just plain doesn't work. It will almost always miss all crossing restrictions by 3-5 miles; unless you have a strong headwind. Consequently; from experience (and from observing the other pilots I fly with; all of whom have considerably more experience on the aircraft than I do); I have learned to descend manually; in vertical speed or level change mode. In this instance; I started to descend in vertical speed with the intention of switching to level change later. I then got distracted or interrupted and forgot to switch descent modes until it was too late; and missed the crossing restriction. Factor #1: Economics. Airlines have determined that it is cheapest to descend at idle; and FMSs are programmed to descend so as to arrive exactly at the bottom of descent point at idle; and then add power at or after that point. Heaven forbid that we arrive and add power a mile (i.e. 10-15 seconds) early. Factor #2: The FMSs in the B737-300s don't work and always miss crossing restrictions. Factor #3: I got distracted and didn't remember to do all of the jobs that the FMS should be able to do in the first place.