I'll give you the distinction on a phone, but on a tablet, I'm really not so sure. What is the fundamental difference between an ultrabook, a Surface Pro, or a more traditional tablet. Sure I'll admit that as you move toward smaller tablets like 7 inch ones, running a full desktop OS becomes cumbersome, but you have to admit that there are some advantages to being able to run a full desktop application on your tablet in a pinch. Sure you'd want to be using tablet focused apps most of the time, but it's nice to know that if you only have your tablet on you, you can still do things like quickly edit an Excel or Word file with real MS Office. You wouldn't want to make huge edits, but just small little annotations would be great.
As you move up to larger tablets, like 10 inch and above, Android and iOS really start to show their lack of features because they don't let you run multiple apps at the same time, and they don't let you do things like mount network drives that every application can have access to. They don't come with standard USB ports, so you can't just plug in a USB stick, printer, camera, or other devices without using special dongles, and most likely they won't have drivers for things like printers anyway.