This isn't an example of realism being chosen over gameplay. Gameplay itself is moving away from every fortress being self-sufficient and having everything it could ever possibly want, and moving towards a greater focus on trade and interaction between your site and others.
The thing is, we're so used to being able to find basically anything we want that it's a little jarring that we can't, especially since the features that are intended to make up for it (like greater trade) are still in their infancy.
This, but another thing:
We still don't exactly have a "starting mission" yet, but presumably, our fortress is meant to expand our little dwarven empires. Now that every mineral and metal in the world is suddenly much more rare, and I find myself much less able to trade for certain materials, just because
not even the mountainhome has any metal ores to trade to me besides galena and sphalerite, then it makes much more reasonable that you be able to actually check what metals your civ does have access to, and what they need, and then embark in a place where you can find (and trade back) the things that your civ doesn't already have to trade up to you.
In fact, you should be able to see what materials any given civ has already prepared to trade with its forts
before the embark screen, because if we're going to be trading stuff back to the "home base" or with humans or whatever, and we only want to be embarking on locations with access to the things the mountainhome doesn't already have, then it's worth knowing what the mountainhome doesn't already have, and what the place we are embarking on will have. That's just some of the basic information that would be necessary to set this sort of system up.
Embarking on a metal-less, or only-useless-ores map can be a challenge and can be fun to play for the same reason people embarked on glaciers before, but the thing is, they knowingly chose to walk into a glacier to set up their fort. People aren't capable of knowing what choice they are making when they walk onto a cinnibar-only or galena-only map versus walking onto a map that rains coal and iron into their lap. It's a total coin flip whether they are playing on "easy mode" or "hard mode", and the player isn't the one making that choice. That's a problem.
There's also the problem of having mountainfaces exposed to open air, here, where it shouldn't take a geological core sampling team to figure out "hey, look! That entire cliff face is made of magnetite! Maybe the ore our metal ore radar that lets us magically know where "deep metal" is and "shallow metal" is is picking up is magnetite!"
Maybe we dont' have core samples, but the result of this change is a stupid one from a play basis - we have to be our own core samplers, and make a savescum save copy, embark, dig down, see what we hit, and then load the game back from the start if we wanted to dig into a copper mine, and instead find nothing but cinnibar. That, or use reveal when the things like DFHack get up to date.
That means the net effect is having to make dozens of embarks and abandoning to get the net effect of something the game should just be able to tell us straight-out. That's pointless, tedious work to someone who just wants to hit "Play Now!", use the finder to get to a site with the amount of features he/she was looking for, and actually Play Now. (Of course, I fully expect some sort of DFHack utility to come out with a geologist function that does a more detailed "find" function relatively soon...)
Even in real life from before modern geology, you typically only set up a mining expedition to create a permanent mining operation in a place where you actually know there is something there worth mining. It's typically worth checking before you irrevocably commit yourself to spending the rest of your life mining that worthless ore vein. (And it also makes it amusing that the spoiler metal seems much more common now than iron.)
On a somewhat unrelated note, what about coal and our dear friend microcline? I find it a little bizzare that I can find low-grade useless gems everywhere and plenty of flux, but no coal, and not even microcline. You're supposed to be able to find the feldspars almost literally everywhere, and I have to wonder if it counts as a "metal" like cinnibar does, just for being in the "mineral" raw?
Suddenly, ninjas, everywhere!