The old adage you get what you pay for applies. Why do you think it is so much cheaper? The fact is, if you are on the HRA/HSA plan and have a serious problem (cancer, HIV, car wreck, etc. something that requires serious expensive treatment) you are looking at $10,000 to $20,000 due in full overnight. Under the HMO/PPO, those amounts would be about 90% less.
Frankly, I think the PPO plan isn't much better. Does anybody know how much it costs every time to go see a specialist? $55. If you've got cancer or some other condition that requires many visits to a specialist, you have to pay $55 each time. It eats up a huge chunk of your paycheck.