I guess it was a cost thing but on the odd occassion the drivers side only drain can be a problem.
I've kinda sideways parked on a steep hill with the passenger side heavily raised and it's then rained heavily. On subsequently moving the car and in all ignorance of the situation developing i've driven off smartly to find the accumulation of rainwater has quickly leveled out, overwhelmend the drain on the passenger side and flooded over the top of the tray!!!!!!!!
I park more carefully after that and have intended to install a smaller diameter drain on the driver side to prevent such accumulations, but i haven't done it yet..
Chris