Lots do have concrete floors in the coop.. personly i prefer dirt its easier on their feet and legs.. concrete is easier to clean for sure but still chickens have been on dirt for thousands of years so why change it.lol... everyone has their own opinion im sure..
So long as you put down enough cushion by way of wood chips, sand, etc the concrete floor is not really a factor on their feet. It is easier to clean, keeps pests from burrowing through. There are pros and cons to wood, concrete and dirt. It may be just a personal preference but you may need to consider other factors such as the area you live..
About 20 years ago my dad built a coop (about 10 x 10) with a concrete floor. We would let the straw build up about a foot before cleaning out. He always had us throw in several bails of straw immediately after cleaning it out so as to be good to the chickens. It worked well.
i have a concrete floor, i dont put any straw or anything on it till winter to reduce frost bitten toes. during summer, u can either sweep or use a leaf blower to keep everything clean. no more smell or flies!!!