At my antenatal classes, the SIDS video recommended to NOT use them at all. Our teacher also said that when the baby rolls and hits something hard, he/she will learn to roll the other way, but with cot bumpers they will not learn how to roll back because they hit something soft.

Also the cot bumpers could get loose when your child starts fiddling with them and then could fall over while the baby is sleeping.

Agreed, dont know why they sell them for if they are not recommended.