If you and I define a word, we must both use that same definition or we are in breach and errors result, software doesn't work, bridges collapse, space shuttles explode, one of us is in breach of contract, etc.
Only if we're dealing with things like programming, engineering and the law. Regular conversations don't have such strict rules. Generally speaking, this isn't a problem. That being said, it becomes a problem when people have different views on things, such as on whether pregnant women should be allowed to have voluntary abortions. This is when dictionary or encyclopedia definitions -can- be a lifesaver, so long as both parties in a debate agree to use definitions found therein.