There are many different shapes and sizes of desktop cases, so some of them will have plenty of room inside.
If you have a 17" monitor or less, you can place the monitor on top of the desktop case and use the case as a stand. If you have a 19" monitor, then it is so big you do not need a stand, so the desktop case will have to be off to the side somewhere.
One thing to keep in mind is that cases with lots of fans can be noisy, so if you have a desktop case on top of your desk right in front of you, you will hear it more than if you have a tower sitting down on the floor.
When I had a desktop case I used it as a stand for my monitor, and I liked having all the drives handy. It was nice to be able to hear when the HD was busy too. Now that I have a tower, I like the fact that it is quieter (out of the way). Alot just depends on where you have your computer set up, and what kind of space you have available on your desk etc.