One issue that bothered me is when the screw fell into the computer case, when the tower is laying on its side. everything falls down and hits the motherboard and can get stuck. I like that when the tower is vertical, when i drop things they fall down on the ground or into the base of the tower where they are easy to pull out, and don't risk falling on the mobo to knock out a resistor or something.
So maybe for butterfingers there is a tangible risk of destroying your computer when you lay it on its side and drop something, like if you are using a tool like a sledgehammer or something heavy (that's a joke, but I have dropped my screwdriver before...)?
Also, is it an actual risk of damage to the card if you let it hang there by its own weight when the tower is vertical? My experience has been that the video card is light enough where you can slot it in sideways, and the screw holes will remain lined up. If there is some wiggle-room, maybe the card will tilt down a bit and the screw hole will be slightly off, but that's easy to address when you finger-start the screw, and can literally align the card using the screw inserted into the card because the misalignment is very slight.
But maybe there are certain video cards that are so heavy they pull themselves out of the slot or break/splinter the PCB of the video card? I just figure the cards are too strong, as are the PCIe connectors on the motherboard? Am I being too naive here, or is there actual risk? Does this come down to preference or how clumsy you are?