As a part of my Biomed degree last year, i had to do a mini thesis on musculoskeltal development in infants. Unfortunately from my observations on the infants, the walker was not doing anything good for their muscular or bone development. One little girl had no control over her legs and it was causing bow leggedness. (Bone formation is affected).
Admitedly she was using the walker excessively. Several hours per day.
There is also a psychological dependence on the walker that can make a child feel lazy in trying to gain independent mobility. Why try to walk when you can zip around on wheels?