Flash is also disturbing; but focus assist is worse, because it happens before the picture is taken — alerting/warning the subject. If you're not shooting people, then it probably doesn't matter. And, very young children often don't seem to notice. Teenagers, however, can be a very different story.
Yes, the focus assist brightens things up enough to ensure that the autofocus has enough contrast to lock onto the scene. But manual focus is another solution.
I've always switched it off in each camera's menus, and I don't think I've missed a shot.
“If you want to view paradise,
simply look around and view it.
Anything you want to,
Want to change the world,
there’s nothing to it.”
—From "Pure Imagination,"
by Leslie Bricusse (b. 1931–) and Anthony Newley (1931-1999),
from the 1971 movie, “Willy Wonka & the Chocolate Factory”