A simple answer is Why not stock them?
They are cheaper to buy than Brown trout by far,thye grow alot faster than Browns like has been said along with the other reasons.
They are a beautiful coloured fish and fight very well in the main and if from good stock are just as good as browns.If I had any choice the one fish I would stock all the time is the Blues,this fish is top of my list for both beauty and fighting qualities and areodynamics in the fight and unlike the rainbow,do tend to take more imative patterns than larger flies.
It has the qualities of both Browns and rainbow all in one fish.