Why OD on 4spd and not just 5spd?

[Tab Julius <tab@penworks.com>]

I apologize if a greater mechanical background for me would have made this 
obvious... but:

I first saw OD on an automatic.  My Toyota automatic has one, as have other 
vehicles I've owned, and I've always thought of it as the equivalent of 5th 
gear (and a gear "over drive", fwiw).

My Saab has 5 speeds, manual, and no OD.  This reinforces my "OD=5th gear" 
assumption.

So, when I had a chance to think about it, I realized that the 4spd + OD on 
MGs didn't fit into the equation.  Is my understanding of OD wrong?  Why 
not just have 5 speeds / stick positions?  Why 4 speeds plus a switch?

Any enlightenment appreciated...

Thanks

- Tab