Probably not enough people take advantage of them. I think it's mainly because they don't want to give up control over some things. They can be changed, but it's still scary to go through.
I have huge asests, but hardly any liquid. If I were to gift my children the 10k/yr., it would help pass the estate. But, I would have to gift property, or acreage. This will break up what I have and I may not be ready to do that yet. I may need it for collateral or to sell for some unknown reason. It's a tough decision to give up control.