I'm guessing from your post that the reasons for creating both is one of the following -
1. Why the hell not
2. Gen3 assets create smaller file sizes than gen8 or gen9
3. More asset options [like clothing, hair and poses] are available for gen3 than for gen8 or gen9
4. You prefer the look of gen3 over gen8 or gen9.
5. Some reasoning I have probably overlooked...
1. Partly correct. I converted G3 characters to G8 before and i just wanted to try the opposite.
2. I don't care. But G3 requires less PC resources, as i know. So, maybe it will be useful for someone.
3. I doubt that, but G3 definitely has many good assets.
4. I doubt, that i can see the difference between different generations models.
5. Anyway, don't think about my reasons. Just use, if you like it. Or don't use, if you don't like it.