The problem with letting calves out on pasture too early is the same as with introducing hay too early -- they need a higher level of nutrition than what grass or hay can provide (alfalfa being an exception).
The calf's digestive system is small and only holds a very limited amount of food. If the calf fills up on fiberous foods that contain limited nutrition, he won't have enough room left to take in enough high nutrition food to keep his growth rate up.
Calves that are on the cow are not only getting the nutrition from her rich milk, they're also getting other components that she produces that help with digestion and general health. There are numerous beneficial enzymes present in raw milk, including the fact that the cow customizes her milk to fight off whatever pathogen the calf has contacted.
The calf raised by the cow had the advantage of still receiving milk until he's 5, 6, 7, or even 8 months old.
The bottle calf doesn't have that advantage, from the start he has to digest a formula that nature never intended for him and he no longer gets that formula after he's 2 months old (younger in some management systems). His nutritional intake has to approximate that of the calf on the cow if he's to perform.
Personally, I do allow my bottlecalves out on pasture, but not until they're off of the bottle, eating well, and gaining -- then they're allowed out.
Ann B