This debate of F-CA vs K-D7 is part of the reason why I adore Trek after all these years exploring the boundaries of Earthly imagination.
One can draw out so much logic from mere basic visuals and demonstrated intentions. The D7 is to me the specialised attack ship and the F-CA a balanced combination of explorer and combat vessel meant to be self-sufficient.
Logic does dictate that instead of producing these balanced ships in wartime, they should instead focus on attack ships such as the Defiant whist using the CA's superior endurance and avionics as a command ship or tender for long range missions. I'm not claiming the Federation should use the Enterprise as a PF tender but it does make sense if the Feds want to keep their big white ships in the line of battle.
Klingon vessels are to me like Russian attack submarines. Very fast, very powerful but wholly geared towards the offensive. As demonstrated in The Search for Spock, the compact size and crew complement of a Bird of Prey did limit the Klingon commander's options, crew comfort notwithstanding - humans and Klingons have different crew support requirements due to society and physiology thus even if the Federation crew does require a lot more creature comforts resulting in a larger, heavier and clumsier combat ship due to the kind of missions their capital ships are sent into (5 year patrols do insist they care for the people well), it does not mean the Federation crew is weaker than their Klingon counterparts.
If anything, the Federation crews would not be under constant threat of death on board the ships, and will be more open to innovation and individual initiative in handling crisis situations. That can mean the difference between life and death in deep space. The Federation ship designs do seem to reflect this trait of versatility.
Sure, the F-CA could be designed better, but Trek was conceived in the 60s and that was the golden age of imagination without bounds. The Federation ship designs became an icon of Trek and the featured characters, thus we have to make do with supposedly impractical warship designs.
But with the opening of the franchise to new ideas we do see that ship designs become more robust and arguably, more tactically logical - see Data's starship-mauling PF/attack shuttle, the Defiant, and Chakotay's tiny little attack boat that was obviously inspired by Klingon designs. Even the Voyager and Sovereign somehow "make sense" in their balance of long range endurance, maximum durability from modular systems infrastructure, and technologically superior offensive and defensive systems making up for the lack of agility and specialisation present in a classic Klingon attack ship which I agree, is the first example of a "proper" multi-role warship.
In the end both Klingon and Federation vessels are beautiful work of art - it's just that the minds behind their designs are different, approaching similar problems with different means.
And were there a wholly Klingon based Star Trek series we would definitely see the same sort of evolution as evident in the Federation order of battle. Of course, with the Klingons mounting multiple-barrel distruptor arrays and massive mixed phaser batteries to counter the threat of quantum torpedoes until new weapons technology came into play, and that constant evolution of warship design based on strategic requirements can lead anywhere!