I guess I'm silly then. Because I'm a breeder and I require a fenced yard. I hope you don't take offense, but your sheltie is very different than most. Yes, shelties are smart... but shelties also have an instinct to chase things. My fence runs along our driveway and the neighbor drive. My sheltie feel very obligated to escort all cars coming and going. They also go after every bird, bunny... and turtle they see. I've rescued countless turtles.
When I was growing up we had a sheltie and not a fenced yard. We lived out in the country so "it was ok.". And, it was ok for several years... until one day, he was gone. We spent countless hours and days searching, and putting up missing posters, even putting some in people's mailboxes. I cried myself to sleep quit a few nights and it was an ache that didn't go away. Two months later we got a call from a lady that had him! I can still remember the joy. He was never left off leash again until we got a fence put up.