I do like the idea of multi screen support, but I can't picture the 2nd monitor doing much more than a support role either such as what's been already described. Inventory, journal, current map, a history of text/damage/whatever messages, etc...
But then there's issues with different resolutions between monitors, and being able to support all those weird cases... plus what if you like having your 2nd monitor vertical instead of horizontal, how would that affect things, etc.. etc..
There are more people using multiple monitors, sure, but it's still a very small market and I'm not sure if it's worth trying to figure out how to support it for a game that wouldn't necessarily benefit from it. It's not like an RTS (SupCom, TA, etc...), where it's definitely a huge help if you have it.