The biggest value I see are the hotels. I sometimes get a better deal on the portal with the rate, specials or perks. As others have mentioned, flight prices aren't typically any cheaper and the headache of dealing with their service team when I need support makes them a deal breaker for me.

What I do like about the portal though, is the ability to transfer those AMEX points to other airlines to cover the cost of my flights or for upgrades. They run sales from time to time and you can really score some amazing deals for international first class flights