I have wondered the same thing with my coop set up. The only thing I think of with this is-what would they do in nature? As long as they have a way to get up and down with lower roosts...then I would think the 5 foot roost is fine. I had something happen with a lil banty of mine-one of my first chickens. I hesitated to mention this as I know many people would disagree with my decision, However, Ahmelia injured her legs, and couldn't walk. I took her to a vet-also worried about Mereks...and the vet said it was from one of the following reasons:
floor to hard when jumping down (I use deep litter method so there was plenty of cushion)
vitamin deficieny (I use pelleted food, give supplements, and lots of fruit and veggie treats)
genetic disposition brought on my trauma (jumping down, vitamin deficiency, not enough vitamin D during winter months, stress)
I believe it has to be genetic as none of the rest of the chickens have had issues. I choose to not have her put down and have kept her in a rabbit cage for almost a year. In the summer I take her out for awhile every day and let her play in the grass, in the winter I take her out just to be held. I hope and beleive that she isnt suffering, and she is suh a sweet chicken. I am sure most others wouldnt make the same choice as I did...Ahmelias feet are useless but she gets around on her elbos/hocks and is full of life. I have had to be very careful that other chickens cant get to her when shes out and when Im not looking, as her toes seem to be fasinating to the others.