As far as gaming goes, there would be three reasons...
You wish to run one monitor at the highest resolution and max settings with no lag in performance.
You want to run a ATI Eyefinity or NVidia Surround setup (3x1 Monitors ie 5760x1080).
You want to run a game in 3D mode. Since 3D essentially requires 120Hz monitors and will basically be doubling the requirements of a graphics card since it's showing two images, you'll need more power.
Also, keep in mind that you will want to check the graphic card's potential, motherboard PCIE lane bandwidth. For example, if you have a video card that will push a PCIE2 lane up to 8x, and when using your motherboard in SLI/Crossfire, it will run at x16,x4 then your first card will be used at it's full potential but your second card would be lacking. There's few cards that would push x8 or even x16 so you probably don't have to worry about it too much, just something to consider.
Side note: To run 3 monitors on NVidia, you'll need two cards. Two monitors plugged into the first card and one monitor plugged into the second card. With ATI's Eyefinity, you can run one card and three monitors. Many of the ATI cards will have a combination of 1-2 DVIs and 1-2 Display Ports. The HDMI port on the ATI card is rather useless in Eyefinity setups.