I had a big long answer ready, but I see that George covered all my points so I've deleted most of it.
Most race results programs have a facility to enter number of laps completed. F18s might do four laps to Dart 18s 3 laps. Before we used this system, Dart 18 and 16 sailors felt bad about making the faster cats wait around between races.
Also, if slower cats knew that there were not going to be many of their class at an event, they might not travel to that event, because of finishing a race way after the rest of the fleet.
Tactics on the beat may suffer a bit, because the fleet has to cross the start/finish line each lap, but it is usually not more than 500 yards to windward of the leeward mark.