As for holding in the LTMA - to gain the best and most efficient use of a runway that has over 90% occupancy rates, you need to have a constant supply of aircraft to vector onto final. That is why Heathrow runs best (most efficiently) when it has a few aircraft in the hold.
There's a clear analogy with manufacturing processes here. 30 years ago, everyone built up huge levels of stock at many stages of a manufacturing process. These days, intermediate parts are produced "just in time", significantly reducing costs. Much of this benefit comes from better information flows within the processes.
To create a constant supply of aircraft to vector on to final, you don't necessarily need holding stacks (stock). But you do need a more joined up ATC system than we have today, with its own form of Kanban.