The Hippopotamus Defence is considered one of the most solid defences and GM Tigran Petrosian is considered one of the most defensive players. Apparently, Petrosian knows about Hippopotamus, but why did he never use this super-defensive defence? I thought this opening suits his playing style perfectly.
The Hippo is passive, but definitely not solid.. Black is allowing White to set up the way they like without posing any real challenge.
There are simply too many ways for White to get a lasting advantage, and given that time controls in Petrosian's time were longer than today, I can't think of any situation where it'd be worth it to play the Hippo.