The Defiant class made sense, but only in its very specific context. In Kirk's day, Starfleet had a slightly more military feel, especially during the last couple decades of the 23rd century (the Movie era, all-red uniform years), because of pressure from the Klingons and Romulans. But Starfleet's best ships were good matches for the best Klingon or Romulan ships, their captains and crews were more broadly trained, and the peaceful Federation was generally much more appealing to neutral parties. By the end of the 23rd century, it was clear that the Federation was becoming dominant, and everyone was forced to reign in their paranoia and mistrust, even Chekov. On the scale of grand strategy, the multi-purpose Starfleet explorer vessels outclassed the dedicated warships.
In the early 24th century, this philosophy is more thoroughly embraced, culminating in the Galaxy class, a master of all trades, well suited to supporting the ever-vaster Federation in every conceivable way. Romulan and Klingon attempts to keep up any sort of arms race during this period seem mainly symbolic, with little real conflict left. The Cardassians start a long-lasting scrap, but there's never any sense that it's considered more than a well-contained and distant border dispute to the Federation (except from the point of view of the frontier colonists living out there).
Then the Borg arrive and Starfleet freaks the fuck out. One Borg ship wipes out 38 of theirs in one battle and Earth is almost lost. Panic! Build warships!
Wait, it's five years later, maybe the Borg aren't coming back, let's slow down the arms race again. What's this? The Dominion? Yet another vast and powerful aggressor that can pop up on our borders with ease!? And their ships, while not crazy Borg powerful, are still clearly more potent than Starfleet's best, and they're available in swarms. Panic! Build lots of warships!
In the short term, dedicated warships like the Defiant class and the Klingons' vessels clearly did make a difference during the Dominion War, holding the line. But the bigger picture is that both the Borg and the Dominion were ultimately tamed by brains, not brawn. The smaller, general-purpose explorer USS Voyager took on the entire Delta Quadrant, Borg and all, almost entirely solo without giving up on the chance to explore as it went, and even the USS Defiant itself was frequently diverted from pure combat duty into more exploratory missions in the Alpha and Gamma quadrants, even as the big war raged. And the Federation was the better for it, despite the Defiant being poorly designed for non-combat work compared with most Starfleet vessels. Even if a purely martial version of Starfleet had been able to defeat the Dominion through force alone, that would no longer be the Federation, but the Terran Empire.
So what do you do with all the shooty Defiants after peace breaks out? A good parallel, I think, is what happened with all the leftover high-performance aircraft and rockets built for World War 2, after that conflict ended. Much of it was scrapped or sold, but a lot of the fastest, most powerful planes found uses in science and engineering, with organisations like NACA, which means those old weapons ultimately fed directly into the success of NASA. The same happened again at the close of the Cold War, with NASA inheriting a wide selection of fantastic (for the time) aircraft. Even today, NASA continues to use surplus and modified military aircraft for their non-military flying. Similar things occurred in the USSR and elsewhere too.
So, picture a Defiant class rigged for high-performance research. On the one hand, it makes a good testbed for new technology, with an over-powered warp core offering plenty of extra juice to run experimental systems. Half the tech brought home by Voyager would have needed further research and development before adoption by the rest of Starfleet, notably the quantum slipstream drive, and it would be wasteful to dedicate an entire large Galaxy or Sovereign, or even a medium-sized Intrepid, to such a specific task. The small, efficient Defiant makes a much more sensible testbed for specific systems, in much the same way that the dinky little Oberth-class Pegasus had been used as a testbed for Galaxy-class technology decades earlier.
On the other hand, how many subspace anomalies have wrecked slower, more delicate vessels? Or nearly wrecked the comparatively big, tough Enterprises? Conventional low-powered science vessels, like the Oberth and Nova classes, are great for most research projects, but sometimes you need something tough, powerful and yet expendable for particularly challenging conditions. For example, the Enterprise-B struggled to survive its encounter with the Nexus, and while the more potent Enterprises-D or -E would have had less trouble in the same situation, you'd also be risking a much larger crew, probably including hundreds of civilians. The smaller Defiant class, with its tiny crew but powerful systems, could get the same jobs done, with less risk. Just ditch some weapons and combat sensors and replace them with specialised research sensors and any other gear appropriate to the situation, and you've got exactly the right tool for the job. The word 'modular' comes to mind.
|Not the right way to do it.|
Both of those roles also play into one of the Defiant class's big limitations, its relatively short endurance. Sure, the warp drive and replicators can keep the crew alive for ages, but it lacks the comforts and amenities that give larger Starfleet vessels their psychological durability on long 5-year missions. It's just not a proper explorer. If instead you're just sending your science Defiant out to perform specific short-term research missions as the need arises, and you can keep it based at a big starbase or station in between research projects, then there's no problem. You could use the starbase's facilities to custom tweak the Defiant before each and every project, optimising its research potential. This also means that it could be reconfigured with reasonable speed back to full combat potential, as needed, and used for routine general-purpose patrols of local space until someone finds something interesting for it to study. Its high warp speed would also make it a fair diplomatic courier or emergency first responder, though these are both short-term, ad hoc roles.
I'm sure some Defiants might stay on dedicated combat duty, just in case, but in general a starship class's ability to stay in service is tied to its potential to adapt to new needs. For the majority of highly specialised Defiants (including, let's argue, the USS Dauntless) to remain useful when compared with the rest of Starfleet's multi-purpose vessels, they would need to de-specialise and show their value in other roles, like engineering and science.
I get the context in which the Defiant class came to exist, but such an un-Starfleet design philosophy always bothered me. Now I think that I can live with it, post-Dominion War. And my personal mental comfort is clearly the most important consideration. As such, I now need to go get some groceries.