Ask HN: Multiple Monitors vs. One Big Screen

I’ve always bought 27” iMacs (or used 27” or larger external displays on GNU/Linux boxes) but I see setups with two side-by-side monitors (or every three) everywhere. What are the pros & cons? Don’t the bezels drive you nuts? Does anyone use a vertically oriented monitor? (Portrait mode). (Side note: remember that CRT monitor from the 1990s for use with the original Mac that you could rotate to landscape or portrait mode?)