Depends really, i personally prefer to have 1 monitor, because i'm in a small room, but a KVM switch sorts that issue out.
You can use it for a number of things, though personally for me, i like to just have 1 PC on, more PC's on = more heat, and this weather now, its just too much.
My 2nd pc however when i do have it wired up normally acts as a file server for the house network (5 pcs when you include the pc in question), or for a few older games it acts as a game server for the lan. Apart from that it stays off to keep the heat down, and of course if you're not really using it, you're be paying more electric for something you look at little.
Everyone is different, decide what works best for you.