I am perplexed about how properly tightened locknuts securing the lead weights could come loose. I can imagine a very small amount of compression of the lead happening over thermal cycles and such, but this?
I strongly suspect that those locknuts were never properly tightened to begin with. Sorta like the fuel pickup tubes in the original RV tanks where a few people didn't tighten them properly and they came off, and then Vans issued a mandatory SB that should never have been needed...
The counterweights on my RV-8A (along with the fuel pickups) flying for 12 years now are still as snug as they were the day I installed them back around 2001.
That said, I guess it doesn't hurt to put the nutplates on allowing the option to snug them down in the future, but if you already closed up your empennage tips (and used properly tightened locknuts per the original plans), I wouldn't lose any sleep over it!