It isn't necessarily about the amount of his appearances, it is more pertinent to allude to his role in the appearance. It may have only been once, but that isn't necessarily going to stop Sakurai from adding someone like Chrom into the roster, the magnitude of the role and the prominence of the actual game are relevant in my opinion.
Palutena isn't necessarily associated with Athena though, Hades is associated with Hades. Again, I feel the desire to express this once more, I am not asserting that he is likely, I do not think that Sakurai will include Hades based on his recognizable name. I believe it to be a factor to consider.
So he isn't essential because he is not in, yet? That is fallacious logic. I suppose Ridley isn't essential because he isn't in, yet? It isn't necessarily a fair comparison, but that isn't the premise I am trying to make overt. He is just as essential as he always was, subjectivity is the relevant notion as of current. Clearly, Sakurai doesn't interpret him as necessary, and neither do you. That doesn't necessarily change the fact that an extremely prominent character interpreted as one of the main antagonists is missing.
Precisely, they are not, but the mere association is enough for the player to evoke an emotion connection to the character. It isn't as though Hades only has his name going for him, his personality was quite enjoyable and I am sure many agreed. He has move-set potential, he is unique, he has aesthetic diversity. His allusion to the Greek God is really only one of the reasons I actually think he is essential. I do believe that it could carry the series if done properly though, not necessarily as many Smash players are already Nintendo fans, but his inclusion would make Kid Icarus an important staple to Nintendo as it would retain the necessary exposure to promote the franchise. I believe that Fire Emblem Awakening is in a similar situation. One could argue that Palutena is enough, but that is obviously where I disagree.
I wouldn't be opposed to Medusa though.