I wouldn't use the human metaphor too extensively here. Yes, computers can act like carriers but it isn't the same problem. The problem with people not being immunized is that it provides a population for a disease to take refuge in and evolve to become resistant to immunizations being used in the rest of the population. It sort of acts like a reservoir to drive the evolution of the disease. In computers though the factor that drives the evolution of the virus or whatnot does not work like that. Humans are active in changing the virus to evade antivirus apps. Therefore whether or not there is a refuge of carrier computers is academic. The viruses will still evolve and they will still spread actively via the vectors setup by the virus maker (believe me, having macs as carriers, passively transmitting a virus is not an effective vector).
So, it is pretty much up to the individual. If you're constantly getting files from windows users and sending those files back out to other windows users (or the same ones) it might not be a horrible idea to get an antivirus so you don't keep reinfecting the guys that are sending you infected files (since they've already proven they're easily infected by sending you said files). It shouldn't really be as dramatized at it is though.