Well, you go there for around a year best I can remember. The point of MMI is to teach you Motorcycles (or Marine mechanics) and only things that will apply to your work as a mechanic. I'd say that if the majority of your curriculum is based towards what you're 'majoring' in, compared to a smaller amount at a 4 year school, that you'll probably come out as knowledgable in the major but probably pretty far back on any of the courses or academics like literature and mathematics and such..
Overall, I'd say if you want to work on motorcycles as a tech that this wouldn't be a bad idea with their manufacturer specific courses after most of their major training is done.