Does not apply to OpenMW as OpenMW doesn't have any Bethesda IP in it. Only end users of Bethesda's games are obligated to follow their game's EULA , which they are doing anyway by playing either Morrowind, Oblivion, Skyrim or whatever at a time in which case they are not mixing assets anyway.
Bethesda's issue was with people porting assets and mixing them up in mods (back porting skyrim assets to oblivion for example). I don't agree with it, but that is their thing. Whatever, no impact on OpenMW.
Take for example if OpenMW supports NIF version up to Skyrim (we already kinda do by the way), that opens up a world of difference in terms of asset creation for people using OpenMW-CS and Blender. All of which has absolutely NOTHING to do with Bethesda, so why tell people that it shouldn't be done? Creating new games for OpenMW shouldn't be limited by the file format they use.
Please stop making stuff up and then telling people what they should or shouldn't be doing. You're not helping.