Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Pretty much every Linux user thinks they're immune to viruses, but they're wrong. Just recently, malware was found hidden inside an innocuous-looking Gnome theme from a reputable site. Users who installed the theme also got several scripts installed as root that were designed to attack internet targets, but it could easily have been much worse.
You see, the problem with thinking that Linux is immune is that sooner or later, something like this happens, and you'll have no protection. Yes, 99% of the time you won't need it. Maybe even 99.9% of the time. But if a virus checker saves you just once a year, we think that's a good enough reason to install one.
Of course, there are other good reasons, not least that you don't want to be blamed in the flame-fest that would result if you accidentally forwarded a virus to your Windows-loving friends. For seconds, even if the virus doesn't affect you, wouldn't you at least like to know that it had somehow made its way on to your precious Linux box?