The main issue with great person dilution is that cities normally try to stop growing when they get close to their happiness cap, they do not attempt to stop growing when they reach their unhealthiness cap, so unless you pay close attention to those messages that say "X city will become unhealthy next turn" (of which only 3 can appear at one time, including combat messages..) you will pump out an excess of great healers. You can of course mouse over the city and it will tell you how much health/happiness it has left before reaching the cap. A really nice feature opera added, which has probably gone unnoticed. Some cities grow too damed fast though (Raitlor is a huge cultprit for this) to catch before they start producing great healers. Perhaps cities can be programmed to stop growing once they hit the unhealthy cap, and healers(along with priests) are not among the specialists the computer uses automatically (you have to tell the city to use them).