One of the reasons P&O appealed to us over Princess was that they used trays, just like Celebrity did.
I honestly doubt it will reduce food wastage, those passengers who pile it high are not going to change their habits. But on recent cruises we have rarely seen piles of food left in the buffet, and for wheelchair users like us, trays enable me to collect breakfast for both of us at the same time, without trays I am going to have to make 2 trips increasing the queuing time.