A dedicated keyboard allows multi-key commands (Ctrl-Shift-= for superscript, etc) that a tablet cannot do.
A tablet with a keyboard can, whether that keyboard is a dedicated attachment (e.g., ASUS Transformers and their keyboard slices), via Bluetooth, etc. Over time, developers with apps needing complex input like this will support such keyboards, just as they do with desktop (and, to some extent, Web-based) app counterparts.
A mouse allows for nested menus with thousands of options. That's a no-go for tablets.
Ignoring the fact that "nested menus with thousands of options" is an awful UI on any platform, this is equally possible with a touch interface.