But then yummyfajitas' comparison doesn't apply after all. When transferring money, the recipient has to wait a day until he can order the next transfer. This is not the case with trading, so this whole "three days" claim seems spurious to me.

I think his point was that the time it takes to be agreed to and the time it takes for the trade to clear are two different things, and that it's OK for those to happen on different timescales.

Having the trade agreed to quickly is a big deal even if it could take days to have any tangible assets to show for it.