This reply is probably more from a cruising perspective;
With the mainsheet tailed at the centre it is always to hand to adjust quickly – or free off in a gust.
If you are hard on the wind in a breeze then you maybe trimming the main continually and it will not be cleated, but much of the time the mainsheet is cleated. So it’s important that you can free it off quickly. This is much easier with a centre sheeted main.
A couple of disadvantages to this arrangement;
The fittings for this tend to be a bit more expensive than a fully aft sheeted main. If you sail more than 2 up then the mainsheet does get in the way a bit
there are some more mainsheet photos in the full article;
http://wayfarer.org.uk/index.php?option=com_content&task=view&id=203&Itemid=140
Hope that helps.