Another consideration (and a point in favor of multiple standard monitors): if you play games, then full-screen games won't look stretched. In a multiple-monitor setup DirectX simply deactivates the auxillary monitors and uses the primary one in full-screen mode.
Not a big deal, but to a gamer it might be.
I use a widescreen monitor, and desite this not a single game out of all that I own are streched. How? Nvidia graphics cards scales non-widescreen resolutions while respecting the aspect ratio so you'll just get black borders on the left/right if you prefer that. The games that does do WS natively on the other hand, looks great and feels much better than what a 4:3 monitor can offer.
The argument that f0dder presents I can understand much better though, but it's all a matter of what you get used to. Besides, I can't really think of any application that I use except for notepad (which I never maximize anyway) that doesn't either offer something to fill the sides width (sidebars etc), or the possibility to adjust how wide you want the readable area to be in some way or another.
Widescreen monitors might not be for everyone, but I would never even consider going back to 4:3 or 5:4, because for me and my use the advantages are far to many to ignore.