The only benefit is to increase FPS. (well you need SLI to run more than 2 displays cause nvidia is dumb... AMD cards can run 6 on a single card). in a perfect world adding a second should double your frame rate, its a little less than that. However crossfire and SLI can have driver issues, occasionally some games don't work or don't work at launch and can have microstuttering. and then there is the heat. So....
If you are buying new generally don't do it if you can get a single card that does what you need for close to the same price. Single cards are simply easier to deal with. Where it makes sense is running multiple high-res displays and a single GPU can't do it OR you already have the single card and are looking for an upgrade.
What Res/how many monitors will you use?
At 1080p I would get a single 7870 and overclock it since it can run everything and a PSU ready for crossfire. then in a year or two when it needs some help (and prices drop) I'd buy a second and crossfire to double performance