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.