Leasing your cat

I'm just curious what you think about putting a cat on a leash? I've always let my cats roam free, but my SIL has a cat she puts on a leash when he goes outside. He's mainly an inside cat, and only goes out in small intervals occasionally. I personally wouldn't do it. But she says when he's roamed free people stole him. She had to literally knock on their door to get him back. I think it's best for him in the city if that's the case. He is a beautiful cat.

What do you all think about tethering animals?