...But since this force would be hopelessly spread out, the individual vessels would probably require much more fighting power than the corresponding "main fleet" vessels in order to have the same effect.It makes perfect sense that the independent exploration, survey and frontier policing vessels would have as much fighting power as those of the main fleet
It could be that Starfleet feels the frontier threats are best countered by sending fast vessels that can assess the threat and alert the heavies of its approach. We never saw Kirk defend the Federation core worlds, after all; his only known attempt at defending Federation assets against a direct external threat was "Doomsday Machine", and there his preferred approach was to attempt to call for help. The rest of the time, he was skirmishing.
In the context of "Ultimate Computer", testing M-5 on a cruiser should be less objectionable from the OP's point of view if cruisers are second-rate vessels. Also, a second-rate frontier vessel would offer a wider range of missions to be tested, beyond "bash Klingon capital ships" and "bombard Klingon planets"; equipping the primary combatants for wider missions would be detracting from their combat powers, and equipping the primary surveyors for combat would have a corresponding effect, but a ship that can afford a little detracting (i.e. is far from optimal anyway) would be an ideal testbed.
Timo Saloniemi