It's not terribly important. In general, if you aren't installing shady programs, you're probably fine. That said, I have Lookout on mine, not so much for the antivirus, but for the location / recovery features.
I agree with YayTech. Take the same precautions as you would on your PC. Don't side load any apps you find somewhere other than the Play Store (or Amazon). Be careful with e-mail attachments. Check permissions before you install an app. Just common sense things like that..
I think it's everyone's responsibility to get AV. Why you ask, to prevent Android from ever becoming as infested as Windows. People used to think they didn't need one with Windows either because they were "careful". Careful people don't have babies either
It's really simple. They don't use very much resources, they will help contain any malware that does get through, and they also have added features such as privacy control, locking down apps abilities to phone home, and lost phone help. The bigger question is why NOT have one.