Just a note, Firefox + NoScript prevented the MacDefender trojan which relied upon popping up a JavaScript window that appeared to be from OS X.
However Safari users were severely affected as there is no easy per-site control of Javascript as NoScript is for Firefox only. Still waiting for such a Safari Extension to appear though.
So there is a definite argument for using Firefox + NoScript, however if your smart enough to install Firefox + NoScript, you likely smart enough not to be tricked into clicking a fake Javascript OS X window in Safari.
If it wasn't for all the other features NoScript protects against, I would just say just be a bit more aware using Safari and use either.
Apple has the benefit, like Microsoft with IE, of that's the browser installed on the machine so it's likely most will use that regardless.
It's those who have graduated to more like poweruser/geek status that may consider using alternate browsers for their certain benefits, added security or features.
In some IT departments, they have everyone using Firefox (or Chrome) regardless of platform (Linux, OS X or Windows) as to make assistance and standardization uniform.