I've barely touched the SP side of ROF (I usually fly coop MP), so I'm passing on the campaign for now because I won't be touching it. Likewise I'm passing on the pistol for now as I can't hit the broadside of a balloon with the gunners in this game, let alone with a handgun.
That said, I think they should make more of this stuff. Everything won't appeal to everyone, but something will usually be good for someone.
Don't feel guilty because you've not bought X number of addons, feel good about the Y number you have, especially if you preordered since as Jason said that helps them even more. Believe me, they know what is selling and what is not, and that will alter their future plans. So don't buy something you dislike (because it will prod them to make more like that) but certainly buy the things you do (to prod them to make more like THAT). It's how things work. I've bought every plane and like all but 3 field mods because I want more of those.
FYI, if they made an MP-capable campaign I could fly coop I WOULD buy that.
The Jedi Master
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Back off, man. I'm a scientist.