Malicious attackers find App Inventor so easy to use they can create their own nasty Android apps quickly:
App Inventor doesn’t give malicious apps any special powers nor access to exotic exploits to attack your phone. But it does make the production of Trojanized apps enormously easy. With only a basic understanding of Android programming, an attacker can churn out tons of malicious apps.
via Mobile Threat Monday: Android Attackers Use App Inventor for Evil.
There is nothing in App Inventor itself that is any different than any other Android app written, for example, in Java using Eclipse and the Android SDK. The issue is that App Inventor lowers the barrier for writing Android apps of all types, including malware. Again, there is nothing in App Inventor itself that is nasty.