Wouter,
Thanks for clarification on cost of carbon in hulls vs cost of carbon beams, it`s more the labour cost of making the beams strong & stiff in certain areas, more than the cost of material, which drives the cost up, if I`m understanding you correctly.
If the general consensus is that the advantage of having carbon beams is only fully realised when they are glued in, stiffening the platform, then a rule prohibiting beams from being glued in should suffice, and I would have no objection to carbon beams being allowed. A rule that basically says that the platform must be able to be dismantled for transportation should be enough in that case. I agree with Darryl, no need to outlaw a material that may become the industry standard in the near future. My point on the carbon hull issue was that if we allow full carbon hulls, and carbon masts are allowed, why restrict the beams in material ? The boat still has to conform to the min. weight of the class, it just allows the builder more control over WHERE in the boat to place the weight.
The problem I see with this is that it WILL eventually make the option of home-building in ply obsolete, probably sooner rather than over a long time, since keeping weight out of the bow & stern & centralising it is far more critical than how much the boat weighs, and influences pitching moments far more than crew weight, which you can place where you want it on the boat, anyway.
The mast rule is quite a good way of controlling an arms race, by having a mast tip weight you have ensured that a carbon mast has no real weight advantage over an aluminium mast, so perhaps a min. weight limit can be set for the beams which would keep things in check. While making this point I must admit that I prefer a setup with very few, simple rules that can be adhered to quite easily, lets not get bogged down with one-design-minded regulations.

Cheers
Steve