As far as I know(and I might be wrong) , Spassky is the only top 10 player that has played the hippo while he still was top 10(No 2 specifically).And he played it not once but twice and not in an unimportant game or against an inferior opponent but in World Championship match against Petrosian(1966).For the record , Spassky lost the match (to be fair not because of the hippo)and 3 years later he won Petrosian and the world championship but this time without using hippo.Spassky used hippo many years later when he wasn't World Champion anymore and not even in top-50.Seems that despite the 2 draws against Petrosian he was never really convinced that hippo is a good system.
I can't say if he is right or wrong , that's a something each one of you must decide on his own, but if you are not Spassky and and your opponents are not players like Bronstein, Tal ,Petrosian , Botvinnik and Fischer , I think hippo is more than fine, especially for blitz.
The gme that seems that made quite an impression to Spassky and made him try the system seems to be a game he played against the high priest of hippo Maximilian Ujtelky ,in 1964 ( 2 years before his first match with Petrosian).
A very instructive game and it is interesting to see how Spassky played the hippo.
Spassky used hippo for first time in the 12th game of the match.Petrosian had a 2 wins lead and had won his last game with white(10th) beautifuly an convincingly against Spassky's King's Indian.Spassky's confidence probably wasn't at his best as in the next game(11th) he didn't manage to prove any advantage against French Rubinstein that was considered somewhat passive.In fact we could say that he escaped with the draw as Petrosian was a pawn up.So nothing seemed to work for Boris and since desperate times demand desperate measures he decided to avoid theory and play something unconventional.
Spassky wins the 13th game with white in a surprisingly conventional Caro Kan and reduces his opponent lead to 1 win.In game 14 Petrosian plays Reti(to avoid the hippo?)and faces one more unconventional line (1.Nf3 Nf6 2.g3 b5!?) and in game 15 Spassky continues the experimentations with a weird Closed Sicilian(with Nh3 and f3!?).In game 16 Petrosian must decide if he wants to face 2...b5!? in Reti or start with his usual 1.d4.Will Spassky try the hippo again?
That was the last time Spassky played the hippo and his decision is indeed weird since in game 22 he preffered Polish defense(1.d4 b5)and suffered a painful defeat that was the tombstone for his hopes to win the match(Petrosian had a 2 wins lead and wanted one draw in the 2 remaining games to win the match).
Why not the hippo?What did Spassky fear?
I guess we will never learn.
Hippo is afterall, a system full of mysteries.