Well, I recently got my beta key from the opt-in(lucky me) and here's one observation.. Protoss is lacking for anti-air. It is really the only race i play and the only race i plan to get better at but the stalkers and sentries are just mediocre against any air unit. And the storms of High Templars are just not good enough for the loss of radius size. I'm really talking mainly about Ground to Air units rather than Air to Air but even then Carriers and Mother ships are weak and void rays take too long to power up. If only they can buff and anti air unit or make a new one!
Phoenix's are good AA. Sure it's not ground to air, but it is your best option. Other possible uses: Killing Queens. Three Phoenix's, one picks up the Queen, two kill it. They can attack Collosi, while not being able to be attacked by Collosi. Sniping Medivacs when Terrans go for a Bio Ball. Sniping Overlords, many zerg either leave them floating about undefended in corners of maps, or in huge piles. Picking up key enemy units in a heated battle. If you can spot a Ghost in the back of a Terran army and pick him up, you can prevent him from EMP'ing your army, which is crucial. They have lots of uses, and I would recommend getting some eventually, and almost any game.
Relative to zerg who have only hydras (tier 2 tech) and queens as ground to air...stalkers, sentries, and high templars can be a lot more practical vs air and ground if used properly
as the others mentioned, the phoenix. that thing doesn't even have a ground attack! its sole purpose is anti-air... although graviton beam does let it sort of disable and kill ground units... in groups. stalker is good vs vikings, at least, so it's decent anti-anti-air. light air (banshees and mutas) are pretty hard to deal with. thors for terrans... not much for anyone else.
