Definitions are important. If you're going to be arguing about something, you'd darn well better know exactly what you're arguing about. Otherwise you and your opponent will just talk past each other, and you'll each be baffled at why the other can't comprehend what you're trying to say. I touched on this Monday in the post about use of the word Literally.
One thing I find funny is that "definition" itself is a somewhat ambiguous word.
The way I see it, there are two basic types of definitions (and probably a few more I'm forgetting, but for now let's just focus on the two). One kind of definition is simply an explanation of what a word means. The other kind is some kind of quasi-authoritative source or specification for what something is.
For example, take the word "sedevacantism": Wiktionary defines sedevacantism (as of this writing at least) as "The belief, held by a minority of Traditionalist Catholics, that the present occupant of the papal see is not the true pope and that the see has been vacant since the 1960s." That "held by a minority" part illustrates the difference between the two kinds of definition.
If that's a definition of the first kind, then the bit about who holds the belief is just a bit of context to help the reader understand what the word refers to. If there was a sudden increase in the number of Catholics who became convinced that the post-Vatican II popes have been false popes, then the definition would become inaccurate. (That's no big deal in Wiktionary's case, since it is a wiki after all.)
On the other hand, if it's the other kind of definition, then if large numbers of Catholics suddenly adopt the belief now known as sedevacantism, then that belief will cease to be sedevacantism, since sedevacantism is by definition a minority view.
When I say that definitions are important, I mean that it's important to make sure the person you're talking to knows what you mean. In fact, I'd say that's the single most important thing about communication, because without it, you're not really communicating at all. So it's probably a good idea to make sure it's clear, when you give a definition, whether you're talking about the fundamental nature of a concept or just the way you're using a word.