Just because you sail a dinghy it doesn't necessarily make you a bad person! 

The basic reasons are as follows.

To answer the last part first.  Cats tend to use rotating masts because they sail much faster than dinghies and can utilize the apparent wind effect produced when your forward speed is combined with the wind-speed.  This has the effect a altering the angle that the air arrives at the sail, moving it to more of the bow.  This is why, in a dinghy on the plane, you can pull in your sail a little more.  However, generally dinghies don't go consistently fast enough to fully use this effect all the time, particularly upwind.

The speed also means that cat sails tend to be cut much flatter than dinghy ones.  Compare a WW1 biplane wing section to an airliner wing section.  The biplane, like a dinghy typical sail, has a highly cambered wing section, creating bags of lift, but also lots of speed sapping drag. But as the thing is not designed for high speed, this is an acceptable compromise.  The airliner on the other hand has an much flatter wing, with much less lift, but much less drag, so can but also needs, to fly faster.  To fly slower and land nicely it needs to alter the shape and add lift. Forgetting about the trailing edge flaps, this is achieved by dropping the leading edge of the wing down to increase the camber.  

The cat wing mast has the same effect.  By allowing it to rotate it increases the sail camber, as the sail is cut with this in mind.  However, the amount of rotation can be critical.  On the 'A' cat, for example, as little as 1/2 inch difference can have a noticeable effect particularly with the mast adding 1.5m to the total area. Upwind, the mast generally needs to be rotated to minimize it's drag, i.e.. point it directly into the wind direction, not your heading, and as the apparent wind and your speed increases, rotate it more fore and aft.

The rotation is used to control the power in the mainsail (usually by a 'Mast Spanner', a bar fixed to the mast foot, facing aft and controlled by a line system).  Generally, more rotation means more lift which means more power.  But too much rotation means being overpowered in higher winds, rotating it too little can risk stalling the sail and loosing drive.   So, by careful mast rotation, the cat sailor will set his upwind sail power to suit the wind strength and his weigh.  Put very simply, the more wind, the less rotation - you want forward speed, not just hull flying.  Downhill, you generally let the mast rotate much more as you usually need a fuller sail.

It will also have more subtler tuning effects when combined with downhaul, outhaul, mainsheet tension and traveller position as it is a powerful tool to changing the sail shape.  And knowing which of the above to alter, and by how much, is what makes people World Champions!

Hopefully this helps prove that cat sailors can think too and are not all hooligans!!

Gordon

'A' Class also ran...

As well as the masts shape adding "area" it also helps the airflow to stay stuck to the sail. This minimizes the "separation" bubble.

The separation bubble is the area after the mast where the airflow hasn't re-attached to the sail. It has two negative effects:

1) It causes turbulence as some air is drawn back towards the mast to fill the bubble.

2) The drive a sail creates is roughly at right angles to the cloth (see fig 5&6 on this diagram)

So if the luff isn't creating drive because of the separation bubble a lot of forwards drive is lost, this is especially important upwind.

Obviously with a skinny mast the separation bubble isn't as big, so the benefits of wing masts are reduced.

As to why they are more common on cats, I think the weight is a big factor,

As far as I know, until recently most wing sections had to be made with metal, which made the masts pretty heavy. Cats are more forgiving to having a heavy mast swinging around above the boat, they have to heel quite a bit before the mast goes over the leeward hull and starts adding to the heeling force.

This is made worse by the fact that, because of their shape, wing masts flex more sideways than they do forwards, which is the opposite to what you want from a mast. So they need to be beefed up even more.

So while cats were evolving with wing masts, dinghy sailors were using bigger rigs and using the stays and spreaders to bend the mast and tune the rig (that's not to say most cat masts don't need tuning).

So by the time it became feasible to make wing masts out of carbon, dinghy sailors wanted masts with specific bend characteristics that wing masts couldn't provide (both by being the wrong shape, and the logistics of having shrouds and tight rigging on a rotating mast was impractical)

The big exception to the rule is the NS14, but I think that succeed because the hull was also very efficient, it was sailed by some good sailors and they were restricted to a relatively small sail area, so it was important to get the most out of that area.

A good example is the Cirrus Icon that is currently being developed, it's based on the NS14, but has more sail area, and after testing they've decided not to keep the wing mast and to use a skinny mast instead.

Note: I havn't sailed any boats with wing masts (other than a Hobie 15, and that hardly counts as that rig is awefull). All this is based off what I've read in books (mainly Frank Bethwaite's High/Higher Performance Sailing) and on the web, so I've probably missed something.