It's not just Stuxnet, as at least one other Advanced Persistent Threat uses that tactic. An APT (likely Russian intelligence) recently used encoded PowerShell to break into the Democratic National Committe: https://www.crowdstrike.com/blog/bears-midst-intrusion-democratic-national-committee/

From that article:

> This one-line powershell command, stored only in WMI database, establishes an encrypted connection to C2 and downloads additional powershell modules from it, executing them in memory.

(As a fun coincidence, they also used py2exe to distribute other modules, which is kinda like a separate interpreter using safe_exec)