here are a few reasons why the change was allowed:
1) It updated the boat, a few years previously the Topper had allowed centremains, so that meant pretty well every small dinghy (Optimsits, Pico,....) had a centremain sheet.
2) It was not just a question of being "trendy". The way you hold the mainsheet and tiller and the way you tack is different (facing forward, swapping tiller behind your back), so if you have learned to sail in an Oppie or Pico, trying a (aft mainsheet) Mirror was a pretty negative experience as you needed to learn to tack again. We wanted to encourage young sailors from other classes to sail the boat, not put them off. In a similar way, Mirror sailors progressing onto 420s, Lasers or 29ers found they had to learn to tack again.
3) In Australia centremains were allowed up to National Championship level, so Australian sailors were keen for them to be permitted at International competitions.
From a racing point of view it is possible to get the mainsail sheeted a little closer to the centreline than with the traditional system. It also puts less tension on the mainsail leech, important if you are trying to "open" the leech by twisting off the top of the sail in a light wind. This comes about because the end of the mainsheet, or the bottom mainsheet block, is located centrally on the transom either attached to a rope bridle, or with a split tail mainsheet.
Some sailors prefer the mainsheet coming from aft, but rig the bottom mainsheet block on a bridle so they get the benefit of being able to sheet the mainsail closer. A good example was 2008 European Champion Chris Rust from Brightlingsea SC as you can see in this photo.
fix the (split) mainsheet tails to the mainsheet attachement points on the transom (where the rope bride is attached). The mainsheet then goes up to the block on the boom and then down to the block on the rope bridle and then to the helm. This gives 2:1 with the split tails running into the top mainsheet block when the boom is sheeted in.
Is this within the bounds of the rules? My Sunday boat is a Flying Fifteen. We have this arrangement to bring the boom on centreline.
The mainsheet is Dyneema split tailed, and led to eyes on either side of the hull. The mainsheet is usually direct ie 1:1, but we have the option of going 2:1.
When the main is sheeted in the two tails pass through the block on the boom, and centralise it, but you could use the five blocks as per rules to make this 2:1.
Any comments welcome
thanks for that, i'm sorry I havn't replied sooner. We have just returned from family visiting in Australia.
Hoping to meet you early on this new season, I may try out the fifteen style main to see if it has any obvious advantages/disadvantages.