Will having a recommendation letter from someone in a different subfield hurt my chances of admission to PhD programs?
Obviously, the better your letters of recommendation are, the better they support your application package.   Impersonal, though supportive, letters may not hurt your package, but they don't help as much as fantastic letters from people who know you well.  

So, not so much "hurt", but "not as much help as it could be", which some would say is equivalent to "hurt"