This much I don't mind, more or less.
This is the difference for the most part. Let me try to illustrate what I'm thinking better (I did this on BG already, lol).
You have a 50 GLD that can turn into a DRK or PLD. You progress with DRK and/or PLD like you would intend them to. You gain skills for that specialization, and those are usable by GLD and all GLD related jobs (though mine was limiting in that only PLD could use PLD skills, and DRK could only use DRK skills).
But let's say, while leveling PLD or DRK, that your GLD grows at an equal or slower rate. So you could be a 50 GLD/50 DRK or 55 GLD/60 PLD. Whatever it is, that much doesn't matter. But the skills you gain for your GLD, and your GLD only (these skills aren't related to either DRK or PLD) could be used on any job. Say GLD got an ability to raise defense from all angles, you could use this on GLD, DRK or PLD.
But! If you were to use PUG now (and say PUG turns into MNK or THF), you would be able to use said GLD ability on PUG, THF, MNK, DRK, or PLD. However, skills you get from MNK or THF, are useable only on PUG, MNK or THF, and not on GLD, PLD or DRK.
Essentially, what it boils down to is you playing a specialization you want, getting skills for that and for the basic job that comes along with it as well as the ability to cross class BASIC skills.
I say all this, because I hate the idea of a class falling to the wayside. Besides that, even with your proposal, if you went from 75 DRK and wanted to repec to PLD, you would be a 75 GLD at some point. I think it makes sense, given what you said if GLD still gains its own skills. The difference being, that these skills are cross-class.
Edit:
And to add a little more on why I like cross-class abilities, it's because that progress feels tangible. The progress I made on MRD can be seen when I play THM or PUG. And while these abilities aren't always super useful or necessary, they do fill up slots and can help you further achieve your role and goal. Gaining defensive abilities from MRD isn't necessary, but then again neither is wearing any gear (I kid), but the fact remains that it's nice to see your progress like that.
And while this should remain relatively low key, it's a good boost to have.