Aery is better than leathal tempo and PTA in lane when kayle is weak and she just pokes. Also in a gang when you heal your jungler with W you also shield them(for example). The thing with Omnistone is that if a champion uses all the runes well to some extend he will recycle them quickly and in lategame you procc runes almost all the time because Omnistone is scalling very well to the late game and it gives you a lot of utility. Lethal Tempo and Press the attack may be better than omnistone in specific parts of the game but Omnistone is well-rounded for Kayle and it gives her strenghts in all parts of the game.

In early game it gives you comet, aftershock, aery, Fleet Footwork, grasp of the undying and predator to poke better, take assists and sustain yourself in lane.

In mid game it gives you press the attack, Comet, Hail of Blades, Glacial Augment and electrocude for stronger poke and trades.

In late game it gives you Lethal Tempo, COnqueror, Dark Harvest and Phase Rush for carrying.

Yes it doesn't give you always what you want at the right time but it tends to give you what suits you better and it also gives options, which Kayle needs because she is a diverse champion with an awful early and a strong late game.