Missions add so much more than a simple deathmatch. It gives focus to what you are doing and allows for specific builds for specific goals.
There's a reason online shooters offer more than just a simple kill'em all mode.
Except the games based on major franchises that somehow skip co-op/campaign modes for some reason....
(**** you EA/DICE).
Of course missions lead to having their own problems, without casualties being a real issue SRS (Suicide run syndrome)*is the bane of mission designers everywhere.
If the system forces you to care about your pilots, things get WAY more interesting.
*Suicide Run Syndrome: The attitude and combat style of focusing purely on your objective, regardless of any casualties. If you lose 6 ace pilots but get the macguffin, then you win. In single mission terms, you've won. In a well written campaign however, you've crippled your forces, winning the battle, but losing the war.