Some games have a lot of "stat" information or a lot of UI clutter that buries the action, like flight sims, you want your dash board on one display and your canopy view on another, or if you have the triple head device and three monitors you can go for "surround" gaming. Theres lots of very good reasons for wanting multiple monitors, its not normally about wishing you had a bigger screen.
My current main monitor is a widescreen as well, alas that doesnt really mean much as its only 1680x1050, so it fits just over half as much "information" on it than my old CRT at 2048x1536. I prefer gaming on the CRT from an image quality point of view but form has triumphed over function and my old CRT wouldnt work with my new workstation. Even with the huge amount I could fit on that CRT I STILL see the point in seperating out across two screen the information you want.
The X-series of games I mentioned can be a real pain to play I have an old saved game where I have over 100 stations and several hundered space ships to guide around keeping the business management \ Fleet control on a monitor to the side whilst keeping my main screen clear for the front windscreen on the ship Im flying in real time would be really great