By looking at the browser screen size stats that we can find online we can see that a large percentage of aspect ratios for desktop sized devices (i.e. those you can't rotate) are widescreen.
I personally struggle to think of many applications outside of watching movies and maybe playing games (I'm not a gamer) where a wide screen aspect ratio is much use.
It's awful for reading any sort of book (hence the Kindle being portrait); not much better for the web (even when only compared to 4:3 ratios); and makes any application with complicated menus trickier due to the loss of vertical screen real estate.
For a TV, it makes sense, but for a computer I don't think I quite get it.
Is there a good reason for devices being designed like this ?