But if there is no winning the war, what is the motivation to fly the different kinds of missions?
fun. maybe even score

(people despise scorebunnies, but love to shoot them down

)
Works for pretty many people. Just think of bomber guys attacking strats... not a very effective way to "win the war", but it's still done.
I'm by no means a furballer in the MAs. I'm pretty much a "strategic" player, commiting myself to certain battles/goals and doing absolutely all kinds of stuff. I do about every kind of mission/combat this game is providing me. Furballing, Buff hunting, Tank busting, tanking, porking fields in a ki-67, bombing strats, running supplies, tanking, sinking CV's, protecting GV's...
But even I stopped "winning" the war long time ago

(Most of the time I'm defending for the outnumbered side)
And after a few years of playing, I have come to believing that pure furballers as well as pure war-winners (aka armchair generals) are both a minority. Unfortunately, both are the most vocal minorities in game. (And as you can often see on this BBS, both tend to be "my way or the high way" kind of guys.)
Another reason is one of resources. A lot of work and thoughts would have to go into creating a working, balanced setup. And we know that the main focus of AH is and will be in the foreseeable future, the WW2 arenas. Yes, i would like to see a full-fledged WWI game with like 90 planes, cities, trenches and so on... but that won't happen.
And third: The war on the western front in WW1
was a static war. That's the main characteristic of it. Pilots patrolled over the same front lines for years.
So my dream is... the addition of just a few items and goals, that will provide variety, enable us to do many more mission types, give us more different kinds of fights, while NOT interfering with the very basic dogfighting characteristics of the current WW1 arenas. Keep the front static, keep the airbases indestructible.
I don't want to see yet another arena where furballers are being yelled at for "wasting the resources" and bomber guys are being yelled at for dropping the FH's and killing the fight.