It really isn't, not at the smaller end of drive density. Large numbers of drives, sure, for density/serviceability reasons, you'll need it, but in the 4/6/8/12 drive range, not so much. And with current case options in that range there are lots of tradeoffs for hotswap - cost, cooling, and so on - in exchange for...not much. Slightly more convenient to exchange drives. You may not have to take the server offline for a few minutes. Big deal.
Just 1:1 on the HDD side, that's all you need. There is room for a lower intake but it'll be blocked by the power supply & cables, and also doesn't really do anything there. The two fans up top in a push/pull are all you need to cool the drive cages.
Motherboard side is a different story - there should be nothing blocking the lower intake, and it has a useful role in blowing cool air over your PCI slots/etc. If you have NVMe + 10GB cards it's a must.