Cost is my main reason for doing it, that and I can't get a pelleted all flock.I decided to keep feeding layer to my birds because it's $6 cheaper... at a bag or more a week, that adds up.. I have yet to see any solid evidence in my or my neighbour's birds that is indicative of damage from the calcium, even in birds that are over 8 years old. I think the management style makes a difference; a free ranging bird would eat less and get less calcium.
Just my take on it; I know many won't agree.