I'm using 800x600 in C# / XNA for compatibility with the XBOX 360, but would not bother with any less than 1024x768 for PC.
It would have to be a seriously ancient graphics card / monitor to not support that resolution. My first ever PC monitor / graphics card in 1998 supported it!
EDIT - Oops, you wanted my views on why some casual games still use it... erm, my guess would be that they are trying to ensure 100% compatibility with as many PCs as possible, so that they can maximize their potential sales.
It would have to be a seriously ancient graphics card / monitor to not support that resolution. My first ever PC monitor / graphics card in 1998 supported it!
EDIT - Oops, you wanted my views on why some casual games still use it... erm, my guess would be that they are trying to ensure 100% compatibility with as many PCs as possible, so that they can maximize their potential sales.