Reading API documentation while I code is another good use case for multiple displays. It gives me a quick at-a-glance ability that I can't achieve with alt tabbing on a single monitor. (I can, it just feels a lot slower because I am constantly having to visually relocate where I was within the document)
I refrain from saying that it is a "MUST" or is "REQUIRED", because I can by just fine on my little 15 laptop, but I prefer working on my dual screen setup.
If I'm going to go with just one larger monitor, a nice tiling window manager is nice. Otherwise, I can just alt-tab to the other screen.
I've been happy with one monitor and alt-tab (CMD-tab on OS X) for a long time, but recently I've realized that the extra monitor comes in really handy for certain things. For me, that's having chrome open in one window and the chrome inspector in the other (this allows me to concurrently see + play with the DOM/css/js without it crowding the display).
When I have two monitors and I'm doing this sort of work, I never say to myself: "Oh man, I'm being so productive". But when I'm without the second one, I certainly miss it and feel less productive.
A 17" display with about 1920x1200 also just works for me. I find lower resolutions very hard to work with.
I am getting a laptop for coding soon and plan to get an external USB display to complement it for this reason.
To the OP; just try using two displays for a couple of hours. You'll never go back.
My setups have included:
Three 19" screens at 1280x1024. This was uber productive. Reference/communication on the left screen, code in the middle screen, test on the right screen.
Now I'm running a 23" Samsung and a 27" Asus. I use the 27" to code and the 23" to read/test on. The 27" has a 1920x1080 resolution but larger type causes less strain on my eyes.
Where do I think I'm going to end up? Buying a 27" iMac and using it as a Target Display Mode for my next Macbook Air. I think I could do it all on one 27".
At my current job I have a 24" and a 19". I'm not sure about the extra 19" and how much that gains me, but I'm stuck in VNC all day so I can't alt-tab like normal folks.
My 22" 1680x1050 monitor at home is a bit lacking sometimes for pixel space, but I manage to get stuff done with it.
I agree with the premise that there are diminishing returns after the 2nd screen, but I personally always use my 13" Macbook screen alongside the cinema display when I'm at my desk.
Being able to have 2, 3 or 4 editor/browser/shell/whatever windows tiled and visible isn't essential, but it's a way of working you miss once you've gotten used to it.
i currently have two 22". (Samsung PX2370). Thinking of switching to one 27" apple cinema. If any one have done same or other way around, please share your experience.
2 million pixels or more? Yes.
More importantly IMHO: Bind as much window management to your keyboard as possible.