Eventually, that makes communication impossible. If people are constantly reassigning different values to the same terms, you may have a conversation where each participant is actually discussing something different, and where the conclusions are therefore irrelevant.
To give you a current example, take REST. When a service says it has a REST API, it tells me absolutely nothing, since I don't know if they mean REST as in Roy Fielding's dissertation or REST as in "JSON instead of XML and some URL templates".
Their ability of communicating with me has been destroyed by people constantly reassigning different definitions to the term.