It's not just about fuel. Having flights fly certain speeds allows more predictability as to the actual arrival times. This allows for an ordered arrival and offload at the hub and integration of the freight into the system. There are a finite number of offload crews, stairs (ever have to wait for them?), etc. If everyone arrived as planned, you wouldn't see any delays for the stairs, or lack of gates being ready. This is a lot larger system than it was 10 years ago, let alone 20 years ago, and a lot more complex. Expect to see even more go into this over time.

The increase in speed to ensure ontime can be ok, but depends on what is going on in the system. You don't actually make up that much time for how much the fuel burn increases. The best idea is to ask GOC if you're running late whether it would help to have you increase speed. They do have the big picture, and will tell you whether it will help or just waste gas.