NAV especially for Mac?

I am already having NIS 2011 on my PC. I am planning to buy a macbook pro next month. I am aware that nav for mac will help detect pc based threats  on it.

I am assuming NAV for mac is primarily to protect users from spreading windows based infections

 

But is it essential to go for nav for mac If I am only concerned for protection (from mac based threads). there are hardly any I guess? (Thats what my friends say, apart from the theoretical possibility)