BMW lightened the flywheel on post-1980 airhead twins, some reckon this gives a more responsive engine (I reckon that makes sense, with b.all flywheel my 'modern' Trumpy is extremely responsive even though it's the 'soft' tourer version), some reckon it makes the gear change much better although that could be something to do with the transmission getting quite a few changes as well.
I've also heard that it was to update the machine to keep it more in the same space as contemporary Italian and Japanese products, so I assume these were 'benefitting' from lighter, rather than heavier, flywheels.
In the Rover V8 world there's also view from respected practitioners that you'll get a better driving experience by lightening the standard production flywheel (unless maybe you're using it in a big old saloon) .
I like my BM with it's old-style 'heavy' flywheel. I like my '56 VH with its CI flywheels (which are bog standard but significantly lighter than a spare set of parallel-pin steel wheels that I've got), and I love the Trumpy which seems to have absolutely no flywheel at all.
So IMHO I think heavy flywheel versus light flywheel is mainly down to what sort of driving experience you prefer.
SG (getting in the bunker...
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)