I THINK it stands to reason that a laying hen would need more calcium than a much smaller non egg laying chick...but that doesn't mean it is harmful The only thing that I could find that looked halfway "Technical" and not just opinion was this:
"Minerals:
The calcium requirement of laying hens is difficult to define. Too much dietary calcium interferes with the use of several other minerals, as well as fat, and tends to reduce palatability. For laying, the recommended level of 3.6-5.0% is adequate in most cases, depending on strain, level of egg production, body weight, and egg mass. Older hens, and especially those subjected to high environmental temperature, may require levels up to and perhaps >3.75%.
Unidentified Nutrients:
The chick has requirements for 40 nutrients, together with an adequate level of metabolizable energy. Some unidentified growth and hatchability factors may improve performance under certain stress conditions. However, with the identification of vitamins and the significant role of many trace elements, many poultry nutritionists disregard the importance of such factors. "
http://vetmanual.org/mvm/index.jsp?cfile=htm/bc/206901.htm&hide=1
It doesn't sound too bad, but who knows what "interferes with the use of several other minerals, as well as fat..." means to a growing chick. If it were a huge cost issue, it seems that several people on here seem to give their chicks layer food without them noticing problems. On the other hand, I don't have that many chicks so I don't think it is an issue for me to keep it separate.
I am not sure that the major egg factories are concerned enough about the health of their chickens to do any major studies on it. After their year or so of egg laying they are going to cull anyway. So what would they care about the long term effect of calcium. And I suspect most breeders don't have the money to fund major studies. It may not be so dire that they drop dead from it. It could be they live less long, are less hearty than they would have been. Who knows? Until we see a legitimate study there is no way to really know the difference. It is very possible that it is all a marketing scheme to sell more products, but since I think that it MAY not be good for them, and I can do it without to much cost or effort, why not err on the side of caution.