I wish I had openings to get some kind of cross breeze in my flat!! I can't open windows easily and if some of the other windows had little openings, it would be easier.
Window restrictors are OK for stopping people falling out etc. They aren't likely very good for stopping intruders etc, which is why i still don't use them to keep windows open at night, sadly. I'm on the ground floor. They might be OK for higher windows, though.
For ground floor, all it takes is a pair of bolt cutters and they'd be gone, and with the sort of window I have, they could reach in and open the rest of the window that way, and be in before I know it. I know angle grinders would make a bit more noise, but even still, they are very very quick at it, and even if I'm awake, the fact that someone is breaking in would scare me.
I'd love to find some way of restricting windows that is safer. You might think bolt cutters etc would be uncommon for intruders to have, but they're not really, as they can cut cable locks, bike locks, etc, which are pretty popular.
However, they would be OK for stopping a child escaping or falling out.