I was told the intention is for traffic to use both lanes right up until the merge. To help people change their mentality, they are now introducing signs which actually say "Please use both lanes when queueing" "Merge here" "Merge in turn" etc
The thought process is that if you have a mile of traffic all in the inside lane it is possible to block an earlier junction. If you have to lanes of 1/2 mile the junction is not blocked. So basically, the theory is that using just one lane adds to congestion.
In light of the above, my PTTGOYN is the HGV drivers who straddle both lanes to block traffic from using the outside lane.