You are very, very incorrect. They can get Trojans and maleware and other nasty stuff, but there has yet to be a virus on OS X. Luckally, common sense is all you need to avoid Trojans. Don't download illegal programs or media and things like that, stay away from shady web sites, etc. and you'll be fine.
I use ClamXav just in case there ever is a mac virus. You never know, right?
It won't help, since no antivirus app can detect a virus that doesn't yet exist. They don't know what to look for.
GGJstudios is correct. Think of any real virus. Countless number of people get sick and even died until we had vaccines and cures to fight it. If a brand new virus came out that was unlike any other virus already known (and a Mac virus would be unlike anything we've previously seen), we'd be powerless to stop it until we had enough research to fight it. Just because you had vaccines and medicine for other viruses doesn't mean you'll be any better prepared than anyone else. Over time we may find a way to stop it, but until then you would be just as screwed as the rest of us.
ClamXav is much the same. It looks for some Trojans and stuff (which as I said before that with a little bit of common sense these should never be a problem). It looks for some Windows virus and maybe even some pre-OS X Mac virus (although I could be wrong about that one but it doesn't matter anway), neither which can effect you. It honestly doesn't do much. It will not protect you from a virus if one were to be created (at least not until the virus is studied and a patch can be applied to the AV software to detect and/or remove the virus).
If/When one is created, we'll all hear about it and I'll be one of the first to download what ever AV software protects against it. But in the meantime, all AV software will do is slow down your machine, steal CPU cycles, and eat up hard drive space. If it gives you peace of mind, they go for it. But I, in good conscience, can not recommend it as it is unnecessary.