I feel like I've been getting into a lot of these verbose debates, lately...
It's a good idea/point, but it can be rebutted more easily the more magic is added to DF.
I've never really liked the notion that magic should be unexplained.
Being as it is DF, I rather prefer the notion that magic is a force of nature that simply does not exist in our world, but nevertheless is an understandable and rational force. Magic that exists in the game is predictable, if not exactly well-explained. Evil areas mean zombies and occasionally clouds that do all kinds of not-good things to your dwarves.
I pushed along a thread on exploring the concept of a magic-based ecosystem, where magic, much like regular ecosystems, is consumed by the "autotrophs" that form the "plantlife" of caverns and form the basis of a magical ecosystem, and restored back by "decomposers" that replenish the magic supply of the area.
To simply say "dwarves are magic" and that therefore, none of their attributes has to make sense is deeply dissatisfying.
Just because gameplay wise it is only a disadvantage at this point, I wouldn't go so far as to calling cave-adaptation a weakness and not a strength. Putting game-mechanics aside, it is as much a weakness as it is a strength - an adaptation that changes the dwarves in such a way they physiologically prefer underground and the dark, even though they can handle light and the outside unless they stay underground for prolonged amounts of time without ever wandering outside.
There's nothing really advantageous about cave adaptation no matter how you might look at it. It is, again, a weakness, not a strength. Dwarves can live aboveground for their whole lives with no ill effects. Humans and elves can live underground for their whole lives with no ill effect. It's just that dwarves that spend most of their time underground will start feeling ill effect if they reach the surface.
Dwarves have advantages and adaptations that help them become better cavern survivors, but cave adaptation is less an advantage and more a vestigial weakness, like a moth's confusing artificial light for the moon and flying into a flame.
Those advantages (short size, high strength, magical strange mooding and trances) are completely separate and distinct from their cave adaptation. They work just fine with or without cave adaptation actually taking place in a dwarf. If they do have a darkvision-like ability, then unless it actually only activates once dwarves become cave adapted (and ceases to function if they lose their cave adaptation), then cave adaptation is nothing but a weakness.
My third point, pertaining previous claims of "everyone should be able to do everything with proper training" - I don't agree.
That isn't the point I was making. In fact, I was making a point fairly similar to yours.
The argument I was making was that saying "dwarves are poor swimmers because when you think of dwarves, you think of mountains, and they don't go out to oceans" is invalid reasoning, but that "dwarves are poor swimmers because they have shorter limbs compared to their more bulky torso" is valid reasoning.
If there is a valid physiological reason for an elf not to be capable of metalworking, it's one thing, but at the same time, if an elf is capable of carving wood or sewing images into cloth, why are they incapable of performing that same precision into carving stone images or statues? How different are the requirements to be a wood sculptor from being a statue sculptor?
By comparison, if an ant-man lacks the eyesight and the mental development to appreciate aesthetics, then it makes perfect sense to say they make crappy artists.