I would say that's an underestimate of the value of a 2nd monitor. If you're a programmer, get a second monitor if you have to buy itself. It's worth it.
I think it depends on the OS. OSX does a good job letting one monitor effectively host several applications. I find that harder to do in windows and the linux desktops I have tried.
Of course, OSX still needs a lot of pixels to do its job, so a single laptop screen won't really help. At my current job I have a large second monitor for my MPB and find myself hardly using the laptop screen for anything.
In Linux/BSD/Unix, it depends more on the window manager (dwm/wmii/ion3/ratpoison/etc nullify the need of 2nd monitor)
I guess the 2 monitor programmers are windows guys (nothing wrong with that) ... and then they migrate to osx/linux, bringing the bad habits and polluting osx/linux over time
your second monitor for MBP, is it used for extending the view for osx, or it's just a cloned view of the laptop screen (let's say via external monitor cable)?
if it's a cloned view, vnc is an old and reliable technique that works regardless of cpu/os used (you can control powerpc mac, intel mac, windows, linux, bsd machines ... in one box)
dual/multi-boot and parallel guys, try to beat that! :D
Ratpoison hardly nullifies the need for a second monitor. It's all about screen real estate. You're working on a tough programming problem, you want a few different files of code, a browser open to some documentation, and some extra emacs (in my case) buffers and/or terminals for various errands. You just want it all to be visible at once. It's much easier to just keep everything there rather than "put it away" somewhere that's not visible. This is true regardless of your window manager.
