Cashback with frequent flier miles allows me to take my girlfriend and me on trips around the entire world on business class. Paying only for Airport taxes.
That leaves me with the following costs:
Yearly fee USD380
Airport taxes 2 pax USD600
Total USD980
For these 980 dollars I get air tickets that retail at USD9000.
Conclusion: Amex is great for when you have a structural demand to use whatever the cashback on offer is as good as this.
Of course it also has insurance and concierge service, but I never use neither.