The way I always look at it is no matter how GOOD my dog may be, I can't be sure of any other dog's behavior.
For that reason, pens and tie outs aren't effective. If I have my dog in a pen or on a tie out, and another dog comes out of nowhere, I now have to try to get to my dog and get a hold of him/her while a loose dog is attacking, trying to play with, or otherwise upsetting my dog. By the time I can get my dog away from the fracas, damage has been done.
With the 6 ft leash in my hand, I can quickly and easily get my dog inside the clipper and away from the other dog.
I have had too many other dogs behave less than appropriately towards my dogs. And I have watched too many other owners ignore or stand dumbfounded not knowing how to get a hold of their dog. I'm not putting my dogs' safety into anybody else's control but my own.