I'm not sure that's "unpleasant design" so much as outright bad design. Aluminum is cheap and easy to extrude into rectangular tubes suitable for use as bench

I'm not sure that's "unpleasant design" so much as outright bad design. Aluminum is cheap and easy to extrude into rectangular tubes suitable for use as bench slats. Unfortunately aluminum is a pretty terrible material for this use. It's freezing cold in the winter, as you noted. It can also be hot enough to burn in the summer sun. And it's hard. And it's ugly (when used this way). I don't think any of this is intended to deter people from sleeping on it. It's just a cheap way to install a bench.