Did anyone try this before? I'd import a few defaults I made into TSRW and save them as a sims3pack file, but I don't really wanna mess around with stuff before at least having asked if it's a really dumb idea. (I do know that the importing defaults into TSRW part works although you can't make defaults with it, I made my designable plants like that. But I'm not sure if they'll work as sims3pack files because I've never seen it.)

The games does NOT read Sims3Pack. When you install a Sims3Pack the content is bundled within file inside the DCCache folder and an individual version of the object is installed inside DCBackup (this is used if you share the lot or Sim, the contents inside this folder will be packed along with your Sims3Pack). You can think of Sims3Packs as rar files, so they contain other files but they have no use for the game. Default replacements should work fine provided they're inside your Overrides folder (or any other folder with a very high priority).

If what you're asking is whether you can share default replacements as Sims3Packs, the answer is probably not. If they have the same resources than something you have already installed through the Launcher, you won't be able to install them. If your content is in package form, then when you install the default replacements using the Launcher, the content will be installed in the DCBackup folder, which I don't think has a priority high enough to be taken as replacement of anything.

In short, NO. I don't recommend ever using Sims3Pack as a way to share default replacements. It just doesn't make any sense.