The ground is perfectly flat as well. It works great for sleeping, laying down, and changing diapers.
Except you get dirtier, you get wetter lying in the rainy puddles, there is a greater risk of rats in a subway station, and you are suddenly visibly demoted from being someone who wants a lie-down on a bench to someone who looks homeless, which decreases passers-by's sense of empathy towards you.
Hey, cool, the Camden bench also dissipates water faster, too!
If you feel ill, there's nowhere to lie down for a few minutes.
You can't change your kids diapers on a bum-safe bench.