It is always a good thing to have letters from faculty who are wholly enthusiastic about your performance. However, if the performance is in a different subject from the one in which you're applying for graduate study, their relevance will be at least somewhat lessened compared to letters from faculty in that subject. One reason that this is true is that the admissions faculty are more likely to know faculty in the same subject personally. Even if they don't, they have a lifetime of training at evaluating people in the same subject: e.g, when I read a striking letter from someone I don't know, I often look them up to find out their research profile.
It may be that my field -- mathematics -- is snobbier than most when it comes to discounting letters from those in adjacent fields. But with the exception of physics, statistics and computer science, if I get a letter from someone in a different academic field, they simply can't directly convince me that the candidate they're writing for will be successful in my PhD program because I can't be confident that they know what it takes to be successful in a (let alone my) PhD program in mathematics. They can convince me that the candidate is a very strong / talented / accomplished student in general, which is certainly a good thing, and what they tell me about their skills in Subject X might make a positive impression on me. I would be delighted to hear that a candidate was an award-winning writer. But this would not be a good substitute for a letter from a mathematician telling me that the candidate has what it takes to succeed in my math PhD program. (And you don't need a faculty letter to list the awards you've won; that information should be included elsewhere in your application.)
So I would say: sure, get the letter from this faculty member in a different subject. But use it in addition to other letters from faculty in your subject, not as replacements for it. (In most programs I know, you can freely turn in more letters than are asked for.) If you really can't find more than two strong letters from faculty in your field -- that's a separate issue, by the way -- then it would probably be better to use this letter in place of a letter from someone who is not going to rate you highly. In my opinion you should not use the letter to replace a letter from a faculty member who will say you are strong even though they don't know you well enough to say anything really insightful about you. We don't need deep insight from every letter: merely being vouched for by someone that we know (at least, by reputation) and respect may be good enough. "Got an A / did well on some projects" is not actually a bad letter from the right person.