I understand the argument that a MP only game can be worth 60. I also know that we have a lot of single player only games that should be less than 60. For me though I look at the total package.
My friends and I usually only play a little bit of competitive MP unless we get hooked. In most situations we only get a night or two of fun out of it.
Coop modes will also get a lot of play out of us. Horde type coop will get less play than more mission based coop like GRAW. Either way, this is a big part of our group.
The Single Player, even if it is 6-10 hours, is just one part that makes the 60 dollar purchase worth it for me.
Unless the game really stands out, removing one of these elements in most fps games lessens the value to me and my friends. When I spend 60 bucks on a game I am taking a risk that I am going to get value out of it. If that game takes out 1 or even 2 of the 3 things I look for in a purchase, I am not going to buy it until it drops in price, or word of mouth is amazing. There have been many purchases that I felt burnt on one or two elements, but sort of got my moneys worth because of the single player, or coop.
Rainbow Six was a game I was really looking forward to. I actually enjoyed the campaigns and the coop stuff on the 360 versions of Vegas. From what I have seen, Terrorist Hunt is not the same, and they removed the single player. I am not happy with this, and will not buy it.
I understand though why companies would love to remove single player campaigns. They probably cost more to make, with voice actors, scripting and programming than the mp parts. I am sure they have lots of marketing that shows that people don't play the sp, and that they could cut the budget in half and still sale 10 million copies. I am saying right now that their marketing is wrong. Rainbow Six will under perform. When the big shots look at all the data and try to figure out who to blame besides themselves (its mobile games, blah blah)....they will completely overlook the fact that they removed the campaign.