Tablets serve the same niche that netbooks do. A smaller machine that is more portable than a laptop or desktop that handles tasks that are needed while traveling or away from your more permanent machine. Also something that is not as expensive as your laptop and won't be as painful if lost or stolen.
There seems to be an assumption by the industry that people want to own just one machine that does everything. What is happening is that they own multiple devices that may or may not share similar tasks, but have different levels of portability. You may have one device that stays at home and one that you take on the bus to work with you. Another may be just for long trips. The hard part is not the form factor, but getting those devices to share data in a transparent and secure manner.
Another reason that Microsoft may be grousing about tablets is it breaks the usage model for Windows. Most windows software wants at least a two button mouse. Click for select and right click for context menus. With a tablet you have no right or left mouse buttons so you have to come up with replacements for those actions. Apple has an easier time converting because they were mostly one button instead of two. (And X windows users have three buttons to contend with. (Though two are just cut and paste.))
I expect that tablets will almost entirely replace the netbook market by 2015. By then the OS issues will be worked out and they will "just work".