I've been using the SP3 in combination with an external monitor from the start (SP2 before that). The trouble was, I couldn't connect two displays (tried a media hub but that was useless) so resorted to a Philips UltraWide 21:9 screen instead (essentially, the size of two standard 4:3 monitors in one). With the ability to drag windows to half or quarter of the screen, it's a perfect solution with less cables and a tidier desk.
That said the new MS Dock comes with two MiniDP outputs, so you have even more options now, and the SP4 will display up to 4 screens. I tend to use the SP3 for the keyboard and use this for Outlook and other 'always open' apps, whilst the external monitor is used for things I'm working on in more detail (presentations, word docs, imaging, accounting, etc).
I received the new Dock today. It all works BUT I can't get the external monitor to work with it. I've called MS Support and they're looking into it but as yet, no solution. Has anyone else had this issue, especially with a ultra-wide monitor? Wondering if it's that? Monitor works fine when plugged in directly to the SP3, but through the Dock, I get nothing :-(