It's not demanded, is just a usefull move. But he is like watterfall, surf, cut, rock smash, strenght, etc. If you really want to use thoose moves in your standarth group of six, some of them could do greath in a fight, however most of them (all of them) is best suited for an HM especialist. A Pokemon that you catch only to use thoose moves. If you not want to use a flying type in your team, just catch one who can learn fly to use it only when you want to "teleport" from one city to another, you'l not gonna do this much, so when you are traveling to the next tow, going to places you didn't visit yet, just leave the Pokemon with the move fly in the box, and take him when you need to go to another city, after that, put him on the pc box again. Also, if you had a flying type in your team it would not be worth teaching him, fly, unless fly is the most powerfull and acurate flying move he can learn. For example, for a Staraptor, would be a waste, since you can learn bravebird, aerial ace and better moves later. Also for Pokemons like noctow, it would not be a good choice since his best stat is special atk and not attack.
Resume: You don't need a flying type on your main team, or a Pokemon learning flying, however is usefull teach flying to a secundary Pokemon, an HM especialist, then you can use only when you really need, like cut, strenght, rock climb etc.