Letter of recommendation from Professor who I have not done research with
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.