Android app markets (market, getjar, etc etc) have been less controlled than iphone or windows phone app markets.. so it was ineitable that badness would creep in.
But I'd be more worried about apps that swipe your data or personal info than apps that would damage the o/s. Really, you should read carefully what permissions an app asks for before installing it. Whay a game would want access to you contacts list ? For example...
Article worth reading on this is:
http://www.pcworld.com/article/221213/keep_malware_off_your_android_phone_5_quick_tips.html
Google had to resort to implementing Google Bouncer to scan the market to get rid of malicious apps.
Read about it here: http://mashable.com/2012/02/03/google-bouncer-for-android/
So, its good sense to have Something keeping an eye on things.
I'm using Lookout on my mobile.. gonna see if it can install on the tablet.
https://www.mylookout.com/
Not just for Antivirus though.. Lookout also helps you locate the device if its stolen.. wipes it of personal data if you want etc etc..
Seeing as we are in an age where we can track our stuff when creeps steal it.. then why not do it?
I also have Prey installed on my laptops... its open source and free...
http://preyproject.com/
HTH
Vida