This is quite an old but common question and I'm super curious about you all; how many screens do you have?
I currently have 2 sdcreens which is way better for my productivity levels but it sure did cost a lot to buy two at once. I feel as though 1 can be enough if it's a big enough screen but ultimately 2 just makes things so much simpler to manage, especially if you're a power user.
I have two screens on my desk hooked up to my computer. I need the screen real estate space to be able to be as produtive as possible. I don't think I'd be able to get much work done on just one screen having to close and open windows all the time. I know some people prefer one screen that is large but I like to have two screens to focus on one thing each side. I'm just more comfortable that way. I have two 27 inch screens. My desk is very big!
I like to have one screen as I find it difficult to concentrate when I have to switch my focus between multiple screens. I can see why people do prefer multiple screens as it makes switching between things easier but my way of working is far easier on one screen I think. I do find that I work productively when I split two windows on one screen though, so my single screen does need to be big!
I'm running two screens at the moment because like others have said, I feel as though having multiple screens is really good for when you want to maximise productivity and only using one single screen can be annoying with managing your windows and using the space you have effectively. Everything just feels to cramped so I like to have at least two which works well for me.