The weight saving is far from massive and is hardly noticeable. Speed isn't really an issue in trials so the saving in rotating mass isn't spectacular either. Every little helps though I guess. If you were to take a single wall rim, 32H and drilled 30 20mm dia holes in it and the thickness was 2mm you'd remove about 51g of material. Even if it was double wall you'd remove 102g. As a percentage its an ok saving over the weight of the rim but when you consider a dual ply tyre that weighs around 1.2kg its not much.