Jumpers don't develop the legs properly to promote walking. They encourage shin muscle strength rather than hip and thigh strength which is needed for walking.
Contrary to what Mommyof2 said they do NOT contribute to bowleggedness -bowleggedness is caused by malformation of the bones due to malnutrition, cancer, or other problems. No activity can cause it. Pressure on the hips, pelvis, and crotch can cause hip dysplasia, but this is more commonly caused by swaddling particularly cradle boards. But wider support in the crotch area, not narrower is what prevents this. That is why double diapering is used to prevent and even cure mild dysplasia, its also why babies that wear cloth diapers are less apt to get non-congenital hip dysplasia.
http://www.answers.com/topic/congenital-hip-dysplasia
In the past, stabilization was achieved by placing rolled cotton diapers or a pillow between the thighs. The child may be dressed in two or three diapers, called double or triple diapering. Both these techniques keep the knees in a frog-like position. In the early 2000s, the Pavlik harness and von Rosen splint are commonly used in infants up to the age of six months to spread the legs apart and force the head of the femur into the acetabulum.
http://www.encyclopedia.com/doc/1G2-3447200154.html
Avoiding excessive and prolonged infant hip adduction, or forcing the legs in a straight position close together for periods of time (as in swaddling) may help prevent strain on the hip joints. Early diagnosis remains an important part of prevention of congenital hip dysplasia.
http://en.wikipedia.org/wiki/Baby_sling
It is a common misconception that certain baby carriers can increase or decrease the risk of hip dysplasia. Some baby carrier companies claim that their carriers reduce the risk of hip dysplasia because the fabric is wide at the base such that the baby's kees are higher than the baby's bottom and spread wide. As a result, some parents are concerned that a baby carrier without a wide base actually increases the risk of hip dysplasia. However, while some websites make assertions about a relationship between carrier position and hip dysplasia (Orthoseek), those assertions are not supported by peer- reviewed scientific studies, because there are currently no published scientific studies that either confirm or refute a relationship between hip dysplasia and baby carrier position. This issue remains hotly debated and peer-reviewed research on the issue would be helpful.
http://www.babycenter.com/404_will-baby-walkers-and-jumpers-help-my-baby-learn-to-walk_6878.bc
Exersaucers, as well as door jumpers, are much safer alternatives, but none of these options will help your child learn to walk earlier than usual.
http://www.safekidscanada.ca/SKCForParents/section.asp?s=Safety%2BInformation%2Bby%2BTopic&sID=10774&ss=Product%2BSafety&ssID=24881&sss=Baby%2BJumpers&sssID=24897
Young children have been injured from baby jumpers when they were not used as intended. A baby jumper is a seat that, is hung from the top of a doorframe with a clamp and allows babies to bounce up and down. While these can be fun for a baby, they can cause injuries.
Here's how to help reduce the risk to your child:
Choose the right jumper
Make sure the jumper is appropriate for your baby's height, weight and age. It's important to follow the instructions. If you bought or borrowed a second hand jumper, make sure the jumper has all its parts. Check Consumer Product Safety, Health Canada (make link to health Canada live) to see if the product has been recalled.
Set up the jumper correctly
Injuries have occurred when baby jumpers are not installed properly. Check the instruction booklet for doorways that can support the jumper. Before you place your baby in the jumper, check that the clamps and straps are secured. Use the baby jumper as intended in the instructions.
Supervise your baby at all times
Do not leave your baby unattended while in the jumper. Limit your baby's jumping time to 10-15 minutes.