I use Lookout on my phone and my tablet (have also used Avast and liked it too). There is malware out there for any operating system. It's rare for Android but any risk is enough for me to install something that aims to prevent infection.
Why not? They don't use very many system resources, they're free to install, and may stop my stuff getting trojaned. It doesn't seen very smart to me to leave the possibility open when it's so easy to prevent.