* Training models can be rather computationally expensive. Especially if your business requires training new models very often, this can be prohibitively expensive to do in ec2, whereas the prediction API solves that for you.
* Just hooking up to an open source ML library isn't the whole story. You still need to do backtesting on different algorithms and do the parameter tuning, aka you need some machine learning know-how. The Prediction API does all this for you automatically and probably uses a much larger set of algorithms than you would bother to test yourself.
* Training models can be rather computationally expensive. Especially if your business requires training new models very often, this can be prohibitively expensive to do in ec2, whereas the prediction API solves that for you.
* Just hooking up to an open source ML library isn't the whole story. You still need to do backtesting on different algorithms and do the parameter tuning, aka you need some machine learning know-how. The Prediction API does all this for you automatically and probably uses a much larger set of algorithms than you would bother to test yourself.