The only legitimate purpose of words having definitions in the first place is to let two people communicate - the purpose of attaching certain syllables to certain concepts is to help transport meanings from one mind to another.

Not quite, I think. I frequently use words in my own thinking, and have much reason to think I'm not alone in this, and if you really think that's illegitimate then I'd be interested to know why. (I bet you don't.) And it sure seems like I make use of the fact that (some) words have definitions when I do that.