I have a leg in each camp here.

Firstly, I wouldn't dream of using fillers in full nitro loads, but then I don't use RE-powders ever. ADI (..or Hodgdon 'Extreme' powders) are bulky enough not to require wads or dacron. There is usually a small air-space, but load densities are probably in the order of 95%. No cause for concern there!

(Note exception: .450 #2 and .475 #2 probably benefit from filler, even with ADI/Hodgdon 'Extreme' powders)

Secondly, most 'nitro-for-black' loads DO need filler or wads, and I strongly prefer saddle-felt over foam or dacron/kapok, as others here must know by now. I have concerns over the longevity of polyurethane foam in the monsoon tropics where I live, as it breaks down to a crumbly or gooey mess in a couple of years when used as packing, or in boot-soles, luggage padding, etc. Wool felt, on the other hand, has been retrieved intact from pharoahs' tombs!

I have also found foam to be 'grippy' on the internal case walls, such that it can be compressed within the loaded cartridge by the powder charge if transported bullet-down, and then remain so, causing air-space between the powder-column and the compressed wad.

This is just my long-winded way of agreeing whole-heartedly with the posts of Bill and others: