The biggest threat will technically be trojans... but to avoid splitting hairs... I prefer to use the term malware, as it can cover many forms of malicious software.
Yes, there is absolutely a chance that you might download and install an app that claims to do one thing, and in the background does something else that they don't tell you about. That activity could be a keylogger, it could be some sort of spy app that monitors SMS messages or call logs, or it could be an app that turns around and downloads/installs additional software.
Google has yanked apps from the android market because they were malicious. I think they yanked two of them last summer.
Has there been a widespread threat yet? No.
Do you really NEED to have it now: probably not
Do some of the AV have useful features besides AV (backup, gps locate, lock/wipe): yes.
Is there some value to these apps, even if there isn't an active threat today: probably.
Do I run one? No.
-SF
