What's the Point of Modding FF6 on Different Platforms?

Each version has its specifications and things to improve. I think the version that has the most potential modding-wise is the PC and GBA version. There's a lot to improve on the PC port but once everything is fixed and tools developed and code documented, it could lead to the most complete or definitive version of FF6 with 2D sprites. It's too soon to tell if how the game is structured will allow more modding than the SNES version though. For now, minimal documentation prevent to do so and it's the same for the GBA version to some extent.

There's the whole aspect of learning stuff too that you seems to not take into account (I state this by the pejorative way you're askiing the question insinuating that the SNES version is the best in all aspects). Developing tools or creating patches for the executable of the PC version is a new challenge the game being coded in another language (C++ -> x86 ASM). It's also just fun to trying to understand how a new FF6 works and code new tools.

Same with the GBA version. You'd have to learn ARM7TDMI assembly which is another game in itself. I think the GBA version is more complete than the SNES one if you have things like sound and color restoration patches. I couldn't explain stuff like longer names patch or people trying to cramp up GBA content in the SNES version if it was not an improvement.

I'm pretty sure if the GBA version had more tools and documentation you would ask the same question about the SNES version. Because in my opinion, people don't mod more the SNES version because it's the definitive one but because it's the easiest one to mod.