Personally I don't really use the Through Deck Cruiser descriptor, because those vessels are essentially carriers.
It's also an euphemism, a designation invented for a specific purpose at a specific point in history. Namely, the Royal Navy felt it wouldn't be given the permission to build aircraft carriers, so it decided to call them by a confusing name. Would Starfleet continue to use a name intended solely for obfuscation?
Well, why not? We still speak of "tanks", after all.
The "heavy frigate" as introduced in the
Avenger class blueprints is another historically ballasted thing: the designation was specific to the early US warships that were much more heavily built and armed than the "real" frigates of the day, and were pressed on to capital ship duties, as the US lacked the resources to build actual capital ships. So basically, since "cruiser" is the modern word for a sailing frigate (the transition happened in the 1880s or so), "heavy frigate" should essentially mean "heavy cruiser". Which is perfect for the
Avenger class, considering how Khan's vessel outgunned Kirk's in ST2. But it's also very confusing, since "frigate" and "cruiser" should be competing terms, not terms in simultaneous use.
One should also note that "frigate" at the time the
Avenger blueprints were made was the modern US Navy preferred term for ships that later in the eighties would be known as "cruisers". A "frigate" at the time was not a small perimeter escort, but the largest of capital ships save for the antique battleships! That was no doubt due to the 1800s Continental Navy precedent, and was a US-specific thing.
Admittedly, we mostly have the British to blame for much of the international confusion. During the World Wars, the Royal Navy and its Commonwealth affiliates had to introduce a wide range of small warships for escort duties. At that time, naval classification schemes were relatively straightforward: the fleets had destroyers, cruisers and battleships, each with a specific role, size and design philosophy, and that was that. Now the RN decided to assign "free" names to its new escorts, raising from the dead a number of sailing ship designations and spreading them with abandon. Hence, we suddenly had "frigates", "corvettes", "sloops", "brigs", "trawlers" and "cutters" that were in no way related to the original usage of the terms. And that nonsense is what Starfleet largely builds upon.
Frankly, I would vastly prefer a scifi show that bothered to invent its own logical designation scheme. But Star Trek still largely remains a 1960s military story (with lots of 1950s military spirit in it), and I have a soft spot for anachronisms...
Oh, and
I also avoid terms like Explorer because I think that's too vague.
"Destroyer" is equally vague. Yet the word has a very specific meaning in naval parlance, or at least used to have until WWII. "Explorer" could be similarly carefully defined in the Star Trek universe.
Timo Saloniemi