Well, I’ve also bought Halo, Battlefield, Operation Raccoon City, Gears of War and Left 4 Dead for the MP just as much as for the campaign. It depends if you enjoy, or even play, the MP portion of the game. If you don’t, I imagine you would feel a bit cheated. I play a lot online so the hours of entertainment easily justifies the cost down the road. I know plenty of people who really enjoyed FOC, but they also play online MP a lot so that is probably why the campaign length did not bother them.
Remember that FOC is also on the 360, which only has a 10 GB disc, so they have to make sure it fits on a single disc for that as well. As for games like Bioshock, the first didn't have MP and the second could be breezed through quickly if you chose to (I did a second play through in less than 10 hours). I think what it comes down to is what you want from the game. I enjoy shorter games as long as they have fun MP and if they don’t have MP, then I would expect a considerably longer single player campaign such as The Elder Scrolls or Dues Ex.