As much as I like to think of myself as a linguistic descriptivist, I run into situations where I think I'm talking Standard English with someone, however we're not actually. Serial miscommunication then ensues until one of us wises up and switches vocabulary.
Since the grammar remains the same, and only the vocabulary changes, figuring out which vocab to use seems non-trivial. Additionally, when different vocabularies come into contact, just using your default vocabulary may frustrate/infuriate the person you're talking with.
For example, take the word 'control'. In academic (humanities only?) english, 'control' has negative connotations (power, overconfidence, failure to consider n-order effects of actions), whereas in operator ( workforce? ) english, 'control' has positive connotations ( not losing control of the car and accidentally killing a bystander ). So 'control' can be viewed as a shibboleth of group membership, where your usage of 'control' indicates your group affiliation.
Naturally, both vocabularies have the same concept, they just use different words. So, it seems an oversimplification to assume speaking the same language destroys our group affiliations; we still use language to provide group identification. It's just not as obvious as before.