Letter of recommendation from Professor who I have not done research with I had a meeting with a well respected professor to discuss issues in the research field (biomedical engineering) I am interested in. I am not seeking to do a PhD with him as I intend to go overseas for my PhD studies. During the meeting I asked him in depth questions about why things are the way in the field, and received good answers. He also suggested potential PhD advisors in the research field who are doing good work. I felt the meeting was productive and he may have a little impressed with me (knowledge of the field).

At the end of the meeting he suggested that he could write a letter of recommendation for me, because he knew all these professors. 

However I had not done any research with him. That meeting was the first time I met him and my only interaction with him. 

I mentioned this to him and he said, that's ok, he can still write a letter of recommendation based on my resume and transcript.

My question is will this letter of recommendation be helpful? Is it a good idea to submit this as a 4th letter of recommendation? Is the worst case scenario that at most the letter just counts as nothing?

Some additional information:  


I have funding for my PhD studies  
I am tangentially related to this professor, he was the boss of my supervisor, but he has sinced moved away and that was the first time I met him.
I may only have 2 good letters, from people I did research with. The third letter is from my programme tutor
Whether it helps or not depends quite a bit on where you are applying and what admissions system is used. But I don't think it will hurt. The personal touch is welcomed when appropriate but it is probably more of a letter of introduction than real recommendation. But that, alone, is good. 


  Here is a student I've recently met that you should take a look at. They have some pretty good ideas for research that seems to mesh with ours. They want to study in ...


In a lot of places this wouldn't do as a formal LOR, but as a message, out of band, to a professor would be useful to you for when you arrive at a new place. It gives you a basis for a first conversation. 

But, in terms of admission to a program, you are probably better served by letters from people who know you better and can attest directly to your background and your likelihood of success in the future. I don't think I'd recommend replacing a letter from this person for one that is more relevant. 

But, in a place in which a professor's role is to hire researchers directly and has a heavy influence on admissions, such a letter would be much more relevant. They get a letter from someone they trust who puts their own reputation directly on the line by writing it. I think it will be strongly considered.