Well, the difference would be that I won't have to duplicate the .js file for each variant. That's what you did with your Ural, if i'm not wrong.
... i use that for another reason, i want all the changes to the engine script properties (whyle im working on them) being usable on all variants, whyle im tampering whyte it, so i ID-d all the parts and made a simple IF scripting for each version bunch bound to a parameter to change them (and also to keep in check, what parts were imported properly n seek of inverted normals (holes in model)) - when its done il make the other variants just copies of it whyte its specific scripts aditions. So its more an convenience whyle im adding new modeled modules and their parts into it. Doe, its true, it may come in handy for some of them (like bumper types), but still, all specific ones will have separate scripts for one simple reason - i cant put the script for using the Air-pressure vehicle and the BM-21 launcher in one single file. It would not only result in 150 000 line .js script, but also a messy work when someone ewer want to make changes in them - simply, potential modders would kill me in sleep. (an you would be surprised how often i get lost in the actual)
... so yes, if its just cosmetic, its nicely useful (mind that textures are handled nicely trough the material files and im sure some way will come to change them on the fly on a single "object" too in time - simply for the usability of that approach). But if there are some simulation or more broad scripting differences between the versions, you better make it in separate ones ... doe, its a really nice thing for external payloads on fighter planes or special equipment on civil ones.