*Don't take this as safety advice*
I have plugged numerous tires, even after tire places wouldn't even look at them, with no problem. As long as cords aren't broken, tread isn't bulged, it has been a non-issue for me. I believe the tire places are either A) trying to sell you tire(s) or (B) are so conscious of liability they won't do a repair.
That being said, if you are the type to wonder every time you drive the car, whether the tire will explode in a burst of glory, I'd replace it and move on. I generally will do 2 at a time, if between 50-90% tread. Over that, i'd do one. Under that, all 4. That's just the way I do it... your mileage will vary.