I approach this 'question' differently for myself, it's not so much that I worry about Trojans/malware for my Mac, I know they exist though, it's more like I don't want to be the 'weak' link in the chain. Let's say I pick up some kind of Trojan/malware/virus for a Windows PC, unbeknown to me I send it to someone who uses only Windows PC's, that makes me the bad person because I was thinking I don't need an anti-virus protection ... I'm working with a Mac no? Why do I need any protection? ... I know the other person needs anti-virus protection too, but in case he/she doesn't, I'm not liable, I did what I could to protect my PC/Mac.

So, yeah ... I use a detector, not only for the small(er) chance I get a Mac Trojan/Malware ... But also to protect anybody I do business with ... Well more to protect anybody in general.

Extra benefit, it keeps my system clean(ish) of tracking cookies and other privacy invading nasties.

I am confident my Macs are secure. I do run Little Snitch to monitor incoming and outgoing data connections, but other than that I don't worry.

The nice thing about OS X is the simple way it handles installing applications and if they ask for elevated permissions. I know that there are vulnerabilities with Macs, but I really didn't have an issue on Windows either because I paid attention to what I was running and where I was surfing online.

+1 for little snitch Its shocking to see how many apps make silent connections.

I think that unless I go to a site online that I should be behind a proxy, or start loading thumbdrives I find on the street I should be ok. That said, I will not upload attachments from emails I don't recognize. Even though I have a Mac, it's not hackproof.