In my experience it works fine even if you've left it off by default. I guess it takes a little bit longer to get it started. (Note that I do have sentry mode enabled, that might help as the Y never goes to sleep.)
I've used smart summon twice in 3 months, and only when there were no other moving vehicles or people around. That's about how useful it is. In a parking garage it drove towards me on the left side of the roadway, even while it was easily wide enough for 2 cars to pass...