I guess it depends on how you define useful.
I don't generally have premonitions; mine is in the form of intuition. I just know things. Since I've learned to pay attention to it, it has become very useful - to me. Not trying to convince anyone at all, cause truthfully, I couldn't care less whether someone else "believes" or not. Their call. But I'll give an example of my own intuition. When I had cancer I had chosen an oncologist. But it just didn't feel right to me. I had a gnawing feeling I should get a different one. So even though it was really the last thing I wanted to do at the time, being so ovewhelmed with just the cancer diagnosis and work, two young kids, surgery, etc., I started searching around. There was one that stood out to me. Why, I don't know, she just did - it was all just a very vague but strong feeling. Anyway, I called this one doctor that stood out and was told (rather rudely) she wasn't taking new patients. Well, so much for that idea. But again, that gnawing feeling came back. Must have her. So I called back a second time, got a nice lady on the phone, and explained that I really felt strongly about seeing her, and I asked her to ask the doctor for me if she could see me, I'd come anytime, etc. After being on hold for a few minutes, she came back and told me the doctor said she'd see me. Great, and gnawing feeling was replaced with peace.
Several months later I ran into a cousin of mine. We never knew eachother too well as there was a 20 year age span between us - him being the oldest and me being the youngest on that side of the family. He had also undergone treatment for cancer the previous year, and lo and behold, guess who his doctor was? Yes, the same one I chose. (And no, there were no communications about it that I would have overheard or anything like that. None. I did not know anything about his illness or treatment, let alone who his doctor was.) He told me that doctor had basically saved his life. And here I am, 11 years later too. So I'd say that was pretty useful. As I said before, these types of things happen to me all the time.
People that people here like to call frauds are useful to people they help. One of my closest friends lost her young son in a car accident not too long ago. She has seen mediums who have without a doubt brought her messages from her son. Those messages have brought her more peace than anything else, so I'd say it was very useful - for her. Most of the people who are non-believers going in say they would never believe it unless it happened to them, etc. So I think it's useless to try to convince anyone.