I've never understood the "one light" visual requirement on a CAT IIIB with DH either. Like you say, A4, it doesn't leave a lot of time at all - and what happens if the "one light" you see (and on which you base your decision to continue) turns out to be, for example, a runway edge light, or something on the airfield periphery?
(Admittedly it's unlikely, such is the accuracy and reliability of both ground and airborne equipment these days, but then you come back to the question - why bother with the "one light" requirement at all?)