Martin,
I'm not aware of any rules or regulations in surfcasting that state you must use heavy weight line and weights, indeed, there are times and places where lighter tackle adds to the pleasure and is possibly more effective.
Fishing line technology is continually improving and stronger / thinner lines are readily available (if you can afford them), So it is only natural for the changes to be reflected in the rules of tournament surfcasting, although the use of ultra thin (and strong) braid line is not approved for tournament use yet.
Any changes to line thickness is to the advantage (or maybe disadvantage) of everyone who competes at a particular weight class. So the big hitters on a heavy line can use the lighter line and be even bigger hitters, it is not a formula which allows the less able casters to catch up with the better casters, but a formula which allows everybody to move up a notch. Thinner lines can also be a disadvantage as well, requiring smoother reel control and better line lay etc., so it doesn't necessarily dumb thing down but requires improved skills to benefit from the reduced line diameter.
Like you, I am somewhat perplexed by the inclusion of plug and fly casting into tournament surfcasting, but there again both methods of fishing are gaining popularity with sea fishermen. So perhaps the "Surfcasting" part of the UKSF name needs re-branding to something more generic which includes all types of "fishing" casting.
I don't know why the likes of Neil Kelland, Paul Kerry and Neil McKellow stopped competing at casting tournaments, but I doubt it was because they though the introduction of lighter lines would enable other competitors to catch up. and they most certainly would not admit it if it was the case.
Anyway, that's enough of the pros and cons debate on light line and it seems we may have to agree to disagree. So, how are you and are you going to (or have you) dust off your tournament casting gear and get out on the field again ?