Yeah, I'll take a 16x9 screen... when 2560x1440 becomes affordable! Wait... why would I ever do that? There's 2560x1600 resolution screens available right now!
In my eyes, a perfect "work" type screen would be 4x3 without question. 4x3 is what I have been using for years for everything from work to play, photos and word docs to Blu-ray and Guitar Hero III. This has always suited me just fine.
However, if I was a die-hard for a wider aspect ratio screen, it would still be 16x10 instead of 16x9, and yes, this even includes movie watching. Reason being this: if you're watching movies on a 16x10 screen, you have a small amount of vertical resolution that goes unused, which is honestly a good thing because if you have a control toolbar pop up, it's not covering any of the video. 60 extra pixels on the top and bottom is plenty to take care of this, but if you don't have that little extra bit of space, when your controls pop up, they cover your video. Being that I use 4x3, I've never had this problem, ever. Of course, all the other reasons others have mentioned (gaming, vertical resolution loss, etc...) also apply to my reasoning that if I HAD to go wide, I'd choose 16x10.
However, I can still honestly state that my screen has more overall usable real estate than even 16x10's. 1792 by 1344 > 1920 by 1200. Add it up for yourself!