You don't need this kind of security apps if you only download popular apps an the Android Market or other officials sources.
If you often install random apps from the Android Market or install pirated apps downloaded from random websites, you should install security apps but that won't protect you very well except if your phone is rooted. Unlike PC antivirus, Android antivirus don't have any right to act on any malicious app without user intervention.
Bottom line, you don't need more security software on Android than on Mac OS X, Linux....