This is murky territory, and most of it is based on opinion and gut feeling.
I feel it's best to simply present you with some facts and observations:
- There is malware for Android, and you can get infected by downloading dodgy apps.
- Pirated apps are high-risk for malware. You get what you pay for, and if you gain some value out of a pirated app, the person who cracked the app probably found some way of gaining value too.
- There will be vulnerabilities in your mobile platform, some of which may be exploited by simply browsing to a dodgy site. This is reasonably rare if you're sensible, but risks go up if you're browsing porn or warez sites.
- Mobile anti-virus is very much in its infancy. You are unlikely to find any product that will really protect you against any significant portion of mobile malware. It should, however, catch the common stuff like Zeus and various other nasties.
- Mobile AV often has the benefit of recommending better settings for your device to increase your security. This isn't really anything you couldn't do by yourself with a little time on Google, but it's nice to have.
- The mobile AV products I've tried have not shown any significant reduction in battery life or performance. They seem to be pretty efficient and unobtrusive. This is entirely my personal experience, so YMMV.
All in all, my personal opinion is that it doesn't hurt to use a free AV package for your phone. I wouldn't pay for one, because I'm low risk and don't store anything critical on my device. If you are a high risk user, e.g. due to browsing habits, or due to the type of data you store on your phone, then you may want to consider a proper AV package. The efficacy of them, however, is yet to be proven.