Well, cost efficiency does play into the part of considering a purchase, especially if it can preform better than a single card for less cost most of the time. Of course, there are risks associated with multi-GPU setups that anyone that has been building hardware or been into graphics card would know. Unfortunately, some people do not understand that. Some games right now can't squeeze much out of multi-GPU setups but that hopefully will change as more and more developers try and get the issues fixed beforehand on games. Unfortunately, Crysis 2 did not do so.
Also, 24 FPS is smooth for people, and usually, some videos don't go above that limit. I can only agree to that statement if it was the minimum FPS that a player would experience. However, there is a significant proportion of people who like me, would rather play at a higher framerate because there is a significant difference for us going beyond 30 FPS. For me, games will tend to cause a relative decrease in response and movement for me that I don't like and although it is tolerable, I can't game at that framerate for more than half an hour. For these people, I hope you understand that we just need that extra FPS for a better and more comfortable playing experience and that it is subjectively necessary for us, but I can't really speak for everyone.
You are definitely right, this is why I said
minimum frame rate

but what you say about comfort is right too. However, you don't need 2 cards to achieve that
