There is no problem with using App Inventor to write your own apps and share them with others. The problem is that App Inventor makes it easy to write any app – and malware authors have begun to use App Inventor to create apps that are malware and might do bad things.
“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. More apps means more confusion, and more opportunities for attack.”
Source: Mobile Threat Monday: Android Attackers Use App Inventor for Evil | PCMag