Depends on the user, in my opinion.
The way I work, I benefit from having two monitors, primarily because that enables me to have two independent sets of spaces that can quickly be switched between, which works very nicely for window management. The way I have things set up, I have a primary monitor where my current focus windows sit and a secondary one where secondary/auxiliary windows (chat, music, mail, etc) sit. It works well for switching contexts.
That said, the secondary display doesn't need to be something as nice as a Studio Display for that. In fact some cheap used monitor from your local Craiglist or hand-me-down from a friend will serve this purpose nicely. When I upgrade my my main display some time this year, the old Thunderbolt Display that's currently my primary will become my secondary.