Wait what? I refuse to believe that you're a master level player and you cannot beat the insane AI. And I don't mean by like cannon rushing or some BS; I mean a "stand up" fight. Unless 'nightmare' is a new difficulty level? I can't find any discussion about it if so.
On the muta discussion: I don't think they're suppoesd to be a game-winning unit. And like any strategy that sacrifices X to get Y, you're vulnerable to things that require X. Bum rushing for DTs is dangerous. Bum rushing for banshees is dangerous. The player needs to make sure he can reach his goal *safely*. With costly units like mutas, that probably means some kind of early aggression to get them on the defensive to cover your expansion and build-up for mutas. I htink there's always value to harassing your opponent... whether that's running speedlings into his main while your armies engage, or picking off undefended buildings with flyers or whatever.
KIAman, beating the hard AI isn't too difficult. It 1-bases pretty hard, but if you hold off the first attack, you're golden. I mean I think the hard AI will expand once, but it only builds like 20 workers total the whole game. If you can beat the insane AI, that'd make you like platinum level, maybe. That said, the CPU doesn't play like most human players.
Also, building up until you hit 200 food and then attacking the *hard* AI is terrible, terrible practice for league games. Unless you're playing against other ppl who are bad, you'll never get to sit around and build up to 200 food. Units that sit idle are units being wasted (exaggeration, but you get the point).