> I'm not convinced its not any less childish screwing around just because it generates a shared vocabulary for people. It's not progress of culture or language, simply an increase of quantity of slang and in-jokes.
Why not both? Culture and language "progresses" (progress implies linearity– might be more accurate to say "develops" or "unfolds") through slang and in-jokes!
Check out linguist John McWorther's work on this subject: http://en.wikipedia.org/wiki/John_McWhorter
In fact it's quite interestingly parallel to how technology often starts out as toys and games (and instruments of war and pornography).