Should I consider getting an extra degree if I plan to go to graduate school in that field? I plan to apply to graduate programs in a field unrelated to my current undergraduate studies. Other questions indicate that this is relatively common, but my question is: is there any value in obtaining a degree in my desired field over only taking relevant courses? 

I think that having a relevant degree could only be beneficial, but talking with advisors and professors gives me the impression that there is no need to have a relevant degree. Surprisingly to me, there is even some agreement that this could be harmful, since I would require an extra semester to graduate. I'm in no hurry to graduate (currently planning for four years) so I don't quite understand their concerns. Am I missing something entirely?

Do I even need to consider getting the extra degree or is it really unnecessary?
I have my undergraduate degree in political science and my masters in computer science. Besides having to take all of the required pre-req courses, which added about a year in a half, I never had any issues. The undergraduate for computer science also included a lot of requirements that were useless for my purposes, so it wouldn't have made sense for me to get that degree, then get the Masters. That would have been way more time and money.