I'll have the unpopular opinion and say that not only do you not need a keyboard wedged in between two joysticks, but also that you don't need to build a keyboard tray and you don't need to spend money on a bluetooth/wireless keyboard. Also, you do not need mouse operation (unless you're using it for game playing). All the setup and administration can be handled outside of play time, and a simple frontend handles joystick input, so no need to incorporate it into the design.
Granted, you may want one if you're playing PC games with controls you can't remap (though the computer desk is better for this, IMO) or you have some other reason you really, really need one (I can see You Don't Know Jack being fun on cabs). But for the most part, I'd opt for a cheap $5 or $10 USB keyboard tucked away inside somewhere instead of spending $60 for something that's really a non-essential part of a working arcade cabinet.