Many or most people feed only layer, and that's what it's meant for. It would be interesting to know his source for saying this. One part of it is definitely not true. Each hen has a set number of eggs, at birth. If she lives long enough to lay them all, she will stop laying. Also, hatcheries breed for egg production, or laying more eggs sooner. Some experienced people feel this breeding has led to more frequent cases of internal laying, which is not curable except by removing the hen's reproductive system, an expensive and dangerous procedure, if you can even find a vet who will do it.
It is true that layer often does not have a lot of protein, and some people prefer flock raiser because ti usually has more. It also doesn't have enough calcium for laying hens so they will go through more oyster shell. At least, mine did when I fed them grower rather than layer, because I had chicks in the flock. Also, if you have a rooster, layer is not necessarily the best feed for him -- but, again, many a rooster his lived his whole life on layer. I have seen research that seems to indicate they lay better at 14% protein than higher.
If you fed flock raiser without calcium supplement (oyster shelll,) I would think you would get more soft or thin shelled eggs, unless they get enough calcium in your soil or forage. I wonder if this person feels this way because they have observed cases of internal laying, and blame the feed rather than the breeding.