Completely agree that applying physics etc. to Pokémon is good fun and a lot more interesting than just automatically waving everything away with "You can't bring logic into a Pokémon game." Obviously it 's a game, and it's not going to make perfect sense in every way and it doesn't have to - but that doesn't mean it isn't fun to try to make sense of it anyway!
I still stand by my point that the new husk /isn't/ just less effective, though. As I explained above, Ninjask actually weighs a lot less /given its size/ than Nincada does - it's less dense overall. If Ninjask's armor were as heavy as Nincada's (and the rest of its body also had roughly the same composition), it would weigh almost twice as much as it actually does according to the Pokédex, because Ninjask is also quite a bit bigger than Nincada. So Ninjask's armor really does appear to be much lighter in exchange for its poorer defensive capabilities - which makes a lot more sense, because developing armor that's both heavier and worse at protection upon evolution would be pretty counterproductive.