I don't know about writing long stuff or focused debug sessions but for QA two big monitors is real physical need. For example I need one or two instances of web interface that I test, several shells for logs, cli and support linux PCs, rdp, two analyzer programs, QC with test cases and stuff, any number of needed docs (specs, plans, general tech publications), some notepad with highlighting and the list goes on. And this is not including two work email clients and two work messengers.

Actually I use two monitors and laptop screen, so in total three monitors and on each I tab between several applications.