I never put any in my display tank, but I had some sneak out of the refugium and end up in the display tank. Not a major concern, but looked terrible because by the time I saw it, it had wound its way deep into the bottom rocks of the stacks. Looked bad, but I don't know that it did anything negative to the tank.
I have a good size chunk in a 20 gallon long tank. I have no sump and I added some during the cycle and my wife really likes it, so it has stayed. It's not my favorite, and long term it will go away, but if it makes my wife more interested in the tank and keeps the tank fed with some live food, then I guess it can stay for a while.