I dislike this arguement for two reasons - one is that it implies low market share will ensure no viruses/spyware - and as such you can "compute stupid". Any OS, regardless of market share, will at some point have a virus/spyware/trojan horse or other problem. Today, there is an exploit that effects almost every browser out there but one - IE. Sure not a virus but a sign no matter what OS your are on, you have to be careful.
Second, it implies that writting a virus that would only infect a small percentage of the popluation is not worthwhile. Remeber, many virus writters like the attention their viri gets! It is largely about ego, ego and more ego. Windows has tons of security holes any script kiddie can exploit them. A secure OS, like OSX presents a challenge.
How much media play would one virus writter get for infecting less than 1000 windows machines? At 10,000 they might get noticed. Write one virus that could even POTENTIALLY infect 1 mac OSX machine and I bet you will not hear the end of it! I can almost promise the first virus that DOES infect at least one mac OSX machine will be on your local evening news. So low market share doesn't play as big a part as some windows fans would like you to believe. Matter of fact, the lack of a virus could actually attract MORE virus writters.
This really has nothing to do with porting to X86, or shouldn't IMHO.