In my humble opinion, the plastic part has absolutely nothing to do with the function of the lug mounting or tuning of the drum. It's sole purpose is to hold the barrel nut or threaded tube into the lug.
Even if the plastic part would fall off or get bumped off (which would be hard to do) it would not affect the lug at all.
So if the plastic part has absolutely nothing to do with function/tuning, and even if it fell out it would not effect the lug at all, how does that relate to " It appears its designed like this incase the barrel nut gets damaged and needing replacement."?
Without ever seeing the lug myself in person, I could tell you the plastic piece is integral to the design, LUGWIG wouldn't put it on for aesthetic purposes, my guess would be its a lug nut retainer. So, if it failed, or fell out it'd seem logical the lug nut and its performance would be effected.