Cloud Nine allows Altaria to revenge kill/force out Excadrill, which is very big in a metagame where sandstorm is so common. The thing is, with Cloud Nine, you are able to prevent your team from being sweeped by the all-powerful weather 'mons, and this is a huge utility considering that both rain and sandstorm are currently two of the most effective playstyles - as well as rain/sandstorm cores on balance and offense being very common due to just how threatening they are. Hell, Char-Y+Venu is a core that is starting to see use on upper-ladder teams, and if SUN is being used on the upper ladder you know that the metagame is shifting towards weather. The argument for Cloud Nine being situational while Natural Cure isn't is baffling as Natural Cure is argubly more situational than Cloud Nine as its utility is far less likely to have any effect on the match than that of Cloud Nine, and seeing as Altaria is going to be used on balance and bulky offense anyway it is more than likely that you will have a Blissey (not Chansey, as Chansey is ass outside of stall) or a Clefable to act as a cleric for Altaria if it doesn't carry Heal Bell itself.