For instance there may be five cardiologists listed in your HMO directory, but if you look closely you will see they all work together in the same office. In exchange for an exclusive contract and the promise of lots of new patients, the cardiology group may offer to cut its usual fees. Sounds reasonable, but this can be a problem if you see one cardiologist and find that you do not get along. Or worse yet, you hate his guts and would not see him again if he was the last doctor on earth. Under ordinary circumstances, you would change to a new cardiologist.
Most big communities have plenty of cardiologists to choose from.
Many groups do not allow patients to switch doctors within the group, and if your HMO has no cardiologists outside this group in your area, then you may be stuck with a doctor you can not stand. Or you may end up travelling to the next city to see one. I have seen this happen more times than I can remember. Also, remember that even specialists can specialize. For instance, the orthopedist who specializes in hip replacement may not be on your insurance plan.