What BlacKat said. No one is quite sure whether admissions committees want you to appear focused (with relevant, science related degrees), or diverse (with less relevant, "fun" degrees). Personally, I got in with a major in zoology, and minor in genetics. I agree that often the minors and majors like this overlap. That is what makes them better, in my opinion. For my minor, I technically only had to take one extra class- I just had to make sure that my zoology elective courses also counted for my genetics minor.
I don't know about other people, but I'm not exactly sure how my GPA could handle an extra 15 hours. Yes, this is only one extra class a semester, but some of those semesters I was stretching myself pretty thin (try taking one more class while in biochem, microbio (and lab), genetics lab, and animal nutrition (a pure memorization course), while also working about 30 hours between 3 jobs). The related minor allowed me to take a few interesting courses (I ended up taking a course completely on evolution to fulfill a requirement, and took a molecular genetics course), as well as have a little extra something to put on my resume.
Are any of these really going to help once in vet school? Maybe not. We probably aren't going to talk about specific theories or mechanisms of evolution, and I can't be sure that we'll need to know exactly how differential gene expression happens. So unless you're in a non science degree, it probably won't do much for you academically. But if it means taking 1 extra course, and having a whole extra "degree" to add to your application, with minimal impact on your GPA, I say go for it.