Completely depends on what you're doing. If you're doing a lot of sustained compute, or doing graphics, then yeah you're gonna want some cooling. But it's a useful little machine for all kinds of tasks which don't cause sustained high power consumption.

Two fried on me. One was just running a printserver without a case. It was in summer so ambient temperature was around 32C but still, you telling me you use rpi 5 without even a cooling case?

I've had an rPi4 running a copy of a forum and server (for reference) in one of the fancy aluminum cases which passively cools for a couple of years now, no issues.

The big chunky aluminum ones do seem pretty good on the pi 4. I had one in the flirc case for a long time and it never seemed to have issues. Obviously adds to the cost though. Also not sure if the Pi 5 works as well in them given its higher thermals, and the Pi 4 didn't exactly run cool so imagine the 5 might throttle occasionally without active cooling.

Yeah I should have been more specific, a fan isn't the only option but you need either a fan or a cooling case. Running them naked is too risky now

I have been using a Pi 4 as a desktop computer for a few years (didn't have anything else) with an microSD card and without any fan, heatsink or case. Haven't had anything problems. Obviously, this depends on your environment, but it worked fine for me.