The full circuit is just a bit wank, basically. All it adds is two straights and two hairpins (boring to drive), thins out the concentration of cars, costs more to license (the organisers have to pay a permit fee to the MSA), you can't see it from any of the spectator points, you'd need more marshals, and we also use it to grid up while the previous race is going on to save time. Good for a track day to ease queuing issues though.
Same thing at Anglesey - there's the coastal and international layouts. The international just adds two straights and a hairpin, boring
The tyres will cut through a light shower if there's some residual heat, you can go very nearly as quickly when it's raining lightly like that - however, according to Mr Datalogger, I went in at the same pace as totally dry (my mistake)