Though, I think the tactical upsides of M-5 prompted Starfleet to try again... just with a human element at the helm and most of the danger ironed out of the design (the Prometheus)
The (iirc) TNG Officer's Manual said that the computer in the Enterprise Dee was a M-6.
I think that when an order like evasive pattern delta, or attack pattern four, is given, from that point forward the computer is largely in control of the ship. The patten not only tell the ship what it can do, but also imposes restrictions on what it can't do.
Narrow parameters.
From a certain point of view they don't completely "trust" the ship's computers, so they don't give it free reign. The example of the Voyager's EMH shows what their computer science can do, and what happen when the EMH's ethical programming was "disconnected" shows the problem.
The 24th century that we've seen holds nearly no robots, lack of trust for an autonomous machine?