Hey, I'm serious about the answers, from watching this in action many times. Sentries prevent 5-6 damage with their Guardian Shields because of the Mutalisks' bouncing attack, and Stalkers can maximize their punishment of harassing Mutas with Blink. The only problem is that, since they're stuck on the ground, you can't chase the Mutas around and make sure to keep them away, so if you don't want to be forced into containment against persistent Muta harass, you gotta build Phoenixes, which move too quickly for Mutas to escape and can move while firing to minimize the damage they take and choose when to engage.
Just don't fly any into a Fungal Growth.
How do you propose you stop an easy contain on a map like Lost Temple then? A zerg would be dumb as **** if they encountered your stalkers and sentries head on. Its trivial to dance between expo and main back and forth and while your blink stalkers keep up (try and get blink stalkers and pheonix, lol), your sentries and guardian shield do not. This basically requires you to split your force, maintain a persistent eye on your entrance to FF any speedling push, get Pheonix, Blink Stalkers, Zealots, and Sentries off of 2 bases and have enough to defend. If you try and move out, speedlings get up in your grill, and before you know it, zerg has 3 base while you have 2. The circle of creep gets bigger, and now your basically contained. This problem is actually amplified if you go pheonix, as it effectively cuts your ground army production, and while it can try and help to get a few of them, the second he decides to get a corruptor, its GG.
Muta Ling against protoss is *extremely difficult* to beat once it hits mid to late game. It basically requires early aggression by the toss to either prevent early FE or do significant damage against the build to stay in. And with the viability now of early mass roaches (ling roach now is so hard to stop), its even more difficult. Look at the success of PvZ in GSL, its disgusting how 1-sided every match was (I think every ZvP was 2-0'd by the zerg by an overwhelming majority, grantd most were roach pushes). This has been met with fairly clear commentary supporting opinion by Day9, tasteless, artosis, and pretty much the entire SC2 protoss forum, and a bunch of threads of TL. The opinion is out, and the majority supports my point.