Given that what many touchscreen tablets (particularly e-ink) offer is a Wacom layer, the advantage is that you're getting far more precise indication of location. The difference between Wacom-in-display and Wacom-as-peripheral is much as with a mouse, in that you have to get accustomed to indicating actions off-screen which translate to on-screen movements, though in practice that seems to be a fast learn for most people. This can actually be better as depending on screen registration and depth of the display elements themselves, "direct registration" often isn't with offsets and parallax.

The precision of using a stylus and pen/pencil grip rather than a mouse, trackball, or trackpad is the primary win.

And yes, cropping is in fact a major function for graphic artists, though painting directly with the stylus + tablet is another option.