A bit in self-interest, but I feel that this would add a lot of capability to modding. Since there would need to be a tag added for each individual token that has arguments (extra butcher objects, for example), this could bypass some of that early on. Plus, some syndromes could take up a lot less room this way.
Advantages:
1. It would allow for wide-scale changing of a creature with no fuss--for example, prosthetic limbs without having to make a new creature for each combination of limb replacements. (If my math is correct, that would be seventy creatures--quite daunting! With this, it would require only 4 creature variations)
2. The creature variation syntax is known and fairly elegant.
3. It would make total body part changes far easier in syndromes, as well as tissue adding--but that would probably count as a full body transformation anyway! See below.
Issues:
1. I have no idea how difficult this would be to actually program, since, well, I don't have access to the code and I really have no idea quite how creature variations work in-code.
1a. Subordinate to the above--you need to make a whole new creature to have creature variations. This makes it seem a bit more complex than I think--in fact, it's fully possible that, to make syndromes work with creature variations, Toady would have to code all the syndrome effects for every tag anyway! It would probably be easier to do that in the first place, though, of course, it would still be a bit easier to just make a creature variation and call that in the syndrome, even if that's identical to using all of the CE_ADD_TAG and CE_EXTRA_BUTCHER_OBJECT or whatever comes up later in the syndrome itself.
1b. Still mostly subordinate--creature variation'd creature may still count as creatures in their own right and, thus, incur a memory overhead from their existence. This might seem minor, until you realize that--using the above example of prosthetic limbs--your 2 castes of dwarves would balloon to 140 castes from the prosthetics. If you include not just limbs, but each individual set of limbs and digits (Foot, foot + lower leg, foot + lower leg + upper leg, left first finger, etc.) it balloons to over 20000 castes, (again, if my math holds right) which I can tell you right now, in my experience, would lead to using more RAM than DF can. This should be avoided. (My math for RAM usage comes from Fortbent, which has 144*30 castes involving creature variations to make 4320 castes total, which causes over a GB of RAM usage--pretty extreme, but not as extreme as this would be).
1c. If 1b is true, then creature variations shouldn't be used at all--just syndrome effects. Oh well. Creature variations would mostly be a convenience, but a very nice one.