It depends on the context. You see, the definition is the meaning of the word. And if two people do not agree on what a word means, it can hamper the proper communication of ideas. Without agreement on what a word means, then that word is no longer useful.

And if people have disagreement on the meanings of enough words, then they might find communication between them very difficult.

Now a straight out flame war type argument over a definiton usually does little good, and the effort should always be made to agree on what you'll use a word to mean, or avoid using that word at all if there cannot be an agreement.

It's all about making sure that the other person understands what you're saying, and is not interpreting it as something different due to words.