I see the point and the necessity of defining our terms. That's what words and labels are for, after all. If I say "Feed banana sit bird" that doesn't make much sense, but form it into a cohesive sentence and it becomes "I feed the banana to the bird that's sitting". The first isn't useful at all, the second conveys something though. So we need to define the words we need in order to communicate to others. Otherwise, what's the point of having an online forum if you don't know that I just sat on my phantom tail and it hurt? You need context to understand me.
As Lyc said, everyone's going to experience this a bit differently. And that's okay since we don't have much research into what is/is not therianthropy. We only have our own individual experiences to compare to others in the community. Yet we still need some way to communicate to each other what
Have the terms and definitions evolved over the years? Yes, words do that and that's a good thing. But the core definitions of those words should stay relatively the same, or else the original words hold no meaning and we loose context and the history of the word itself. Basically, you can't just take an existing word and use it for a completely different thing: else we'd all be perpetually confused and English is already confusing enough!
The issue I see more that's the bigger problem is that people are finding these already defined terms, figure it's not quite what they're experiencing, so they create a new term that sort-of-like-the-original-but-not-exactly. Hence why we have all these new, similar terms when we don't need them. People need to understand the terms we have, and if they match them (if even only a little bit) then fine. And if not, then fine, don't use the words to describe what you're not.
Honestly, if even one online community could agree 100% on a definition for therianthropy we'd have a leg up in explaining ourselves to others in places like Tumblr. The issue is that it's such a varied, hard to define concept that even one large community can't pin-down the exact definition for it.