The idea is that the double white line delineates that one direction is viewed as the 'important' side getting the open two lanes, whilst visibility is still acceptable to allow traffic to overtake on the '1' side of the line as well with care.
They aren't meaningless, they are safer than the old S3s in that the middle lane is given more prominence to one side, thus removing the 'chicken' element that plagued S3s.
So lets see if I've got this right: what the road marking mean is if I'm going along the side with the solid line and something is coming the other way I have to let it go through, but if I'm going the other way with the broken line, I don't?
Maybe I'm missing something...
As explained by Glen here, this marking scheme isn't a 3-lane road with centre lane priority. The double solid/dashed line is a centre line, with two lanes on one side and one on the other.
...the "uphill" lanes on a S2+1 are divided by lane divider markings (2m/7m), not centre lines (3m/6m), the same marking as used on dual-carriageways - ie they separate two lanes travelling in the same direction.
Therefore the middle lane is an "uphill" overtaking running lane, whereas traffic on the "downhill" side has to cross a centre line (a DWL broken on their side) to overtake in the same way as overtaking on an S2.
Hmm, read that, clear as mud. Nope, still doesn't make sense.
As far as I'm concerned, I overtake when nothing is coming the other way unless I'm not allowed to by a solid line on my side of a dual line marking. This sort of marking allows overtaking in both directions on an S3 and is in effect exactly the same as two rows of single dashed lines.