Most people new to chickens don't understand the 'dual purpose' vs. broiler concepts.
Those dual purpose birds where what people used for meat birds over the last century. They were big birds, layed good eggs and when spent still offered something for the family table. The thin breasts is actually what is best for a fryer. The chicken cooks through to the bone before it dries out or starts to burn. Reading these birds described as 'dual' purpose isn't so that you can raise them for one purpose (to lay eggs) OR the other (for meat). You raise them for eggs and when spent you take all your hard work and dollars poured into feed and reclaim it as a table bird. Trying to raise these birds and trying to make them into the concept of what our generation has grown up thinking a meat bird should be is throwing good money away. They will never be what a broiler is.
The meat birds were commercially developed and raised and flooded the markets in the 70's. They were selectively bred to be bigger and meatier and made it seem more feasible for the current fast paced lifestyles to just buy a chicken that was cheaper and offered more meat than one that could be raised up at home. This is where our generation (40 somethings) and beyond grew up not knowing there was generally anything about chickens not like the ones in the grocery store.
It is now that so many are flocking back to the old ways, taking responsibility for their food and sustaining their families in the old traditions that all things old are becoming new again.
All sorts of birds have been developed to use as a broiler. Alot of the small guys got tired of being left out and branched out to develop their own versions of the meat birds that are held almost as a industry secret with the commercial breeders and producers. Cornish crosses are not genetically altered. They have been selectively bred for their features - broad breasts, thick thighs, meaty legs. The commercial producers have selected birds that grow bigger, grow faster and have spent the last 50 years perfecting their breed of 'cornish cross' that is nothing like the original birds used in the initial breeding programs.
For me the choice of cornish crosses as opposed to say the ranger lines is I prefer for myself the whiter skinned chickens without pigment tinting from the feather shafts. I grew up in an area where the chickens came from basically one place. All of my life we ate Claxton Chickens. I didn't even know what the names Tyson and Perdue meant outside of a frozen meat case. We were never exposed to those nationally known names as fresh chickens. We shopped at Piggly wiggly and Big Star or Bi-Lo. All of the chickens sold were white skinned. As I developed my own cooking preference, even when processing my own roosters or spent layers, I found if I wanted a roasting bird the white skinned (white feathered) birds looked best - more appetizing to me. The cornish as I raised them were always healthy. Never had troubles with them. They grew out quickly, full breasted, white skinned (not as white as I want but white enough) and look beautiful in my dishes.
They are my preference for a single purpose meat only bird.