Space savings depends on situation though. If you have your desktop case underneath your 19" monitor you're using less desk space than if you have the monitor and computer standing side by side.
Last couple years I've started to miss desktop style cases. Not enough to actually buy one, but enough that I gutted/dremeled and otherwise abused an old 286's case to make it ATX complient enough to work for my spare computer. Then put it on the shelf along with the stereo components, fit in much nicer than a tower there.
Many people have their computer on the floor or a few feet away from the monitor these days. So the footprint usage varies. Overall though, more people can use a tower style without hassle than a desktop. So supply is greater on them, which affects their availability, model selection, pricing.