I don't understand why Apple doesn't produce a wireless version of the number pad keyboard.
...because they don't want to make too many permutations (keypad/no keypad/wireless/no wireless) and they've chosen the two sensible options: the more portable mini- keyboard is comfortable to pick up with one hand and makes more sense as a wireless device. If you want the full-size keyboard (not comfortable to pick up with one hand) you're probably using it on a desk and having a wire isn't going to kill you.
Its nicer not to have batteries to change, there's no problem with boot options (BootCamp, rescue partition etc.) and the USB hub in the wired keyboard is useful (just right for a micro-transmitter for a wireless mouse - and there's a reason for not wanting a trailing wire on a mouse).
If you don't like the choice Apple have made for you then there are third party options (The logitech solar keyboard looks great - but my wired keyboard works without batteries too!)