The Ace Combat games are fun because they don't bother you with sub par control schemes (ever try playing Janes without a joystick? Not fun at all), have terrain that doesn't look like it was pasted from a photo, and give you an engaging storyline to keep you occupied. Sure , having infinite ammo and fuel is a bit unrealistic, but let's be honest, with the odds you're up against in the game running out fuel at a critical moment would just ruin the feel of the game.
A flight game doesn't have to be realistic to be fun, it just has to be good enough to make me feel like I'm flying the plane.
I'm definitely looking forward to Ace Combat 6, and I'm glad they made the smart move to put it on the 360.