You have a use for it, I don't. To me it's a gimmick. My phone is for making calls. I don't need to make them on my Mini or iPad.
If your phone is for making calls, I sort of question the entire argument. You don't need a data plan for calls.
Moreover, your premise of "why use my WiFi when I have LTE" is just as easy to flip around. Why bother using LTE when I have WiFi?
That's the crux of the issue. If the only thing that the unlimited LTE gives you is the ability to use it instead of another available data connection, then it is not an advantage. It becomes a circular argument:
"Why do you have unlimited data?"
"Because I use 40gb per month."
"Why do you use 40gb per month?"
"Because I have unlimited data."
Pretty much everyone in this thread touting the supremacy of unlimited plans is doing so while tacitly admitting that they would see absolutely no difference in their daily life if they did not have it. If you and I go through our days and the exact same amount of data goes through our phones, except mine is through a WiFi connection and yours is through LTE, then in the end there is no reason to suggest that one is better than the other.
That's the weirdness of this whole argument. I could understand people flogging about how they NEED unlimited data if they were constantly on the road, away from WiFi, and doing heavy data tasks. For example, let's say you're on the road for several hours a day for work and streaming music is very important, and it's not hard for you to burn through 5 gigs of music a week. That makes sense, because there is no way for that to happen without that LTE connection.
But if you're sitting at home, and I'm sitting at home, and we're both doing everything we could possibly want to do with our phones, the fact that you're doing it on LTE offers no advantage to me.