Most files don't have resource forks though, so the ._$filename doesn't store resource fork data in most cases. It usually just stores metadata.
I really doubt that 99%+ of Mac users use flash drives / zip files, though. I suspect that well more than 1% of users never use anything like that. A lot of people only have a single computer and share through e.g. just mailing files to people or using Dropbox, or not even that.
While resource forks are deprecated and on the retreat, extended attributes are a new hotness, used extensively (just download a file in the browser, and it will get com.apple.quarantine and com.apple.metadata:kMDItemWhereFroms). These are also shoveled into AppleDouble files. Actually, resource fork is just com.apple.ResourceFork extended attribute.
I'm not sure that there are more than 1% of mac users do not exchange files with other people, possibly on other platforms. What do they use the computer for, then? iPad would be more suitable then.
I think that there are well more than 1% of users, maybe 20% or higher, that never share files except to attach a picture to an email, upload it to Facebook, or something similar like that. It's a bit of a nitpick, though, but we do often forget the "non-power" computer users.
I really doubt that 99%+ of Mac users use flash drives / zip files, though. I suspect that well more than 1% of users never use anything like that. A lot of people only have a single computer and share through e.g. just mailing files to people or using Dropbox, or not even that.