The fast belt increases throughput. In factorio you generally want to maximize throughput, as the investment in the intermediates is fairly minimal. So while a slow inserter may not grab, it just means there is something at the end of the belt that gets resources that otherwise wouldn't have.ratchetfreak wrote:
but that requires the belt to be clogged first. Which means that using a fast belt there will not have any benefit anyway.
Mixing the belts is definitely the way to go for compactness. Yes, the belt will run out faster - but you have tons of extra room to run even more belts to replenish it when it runs out. Off the top of my head I'd estimate you're looking at 3-4x increase in space efficiency - definitely worth it.