I think this one is really a horses-for-courses situation. For me, my current setup of two 2560x1440 monitors plus a 15" laptop is absolutely optimal - I can spread a load of stuff out everywhere, flip between a few spaces, and just generally be super-productive. I can still be productive working from just the laptop in a coffee shop, but I feel very constrained and find myself shuffling windows around to get stuff done (even just writing an email whilst referring to something else), and I generally don't like it and pine for my multiple monitors, especially if I'm doing any sort of development work.

Other people seem to be able to be just as productive from a single monitor or laptop, and tabbing between things is very much just part of their workflow - I remember at one gig we bought a couple of displays for one dev and he never even bothered plugging them in.

(I don't think anyone really "needs" five monitors, though.)