I don't think the glass cup is so that they learn it breaks - I think it's just an ideological thing of glass being nicer than plastic to drink out of, and having the same amount of respect for a child as an adult. We don't typically drink out of plastic, do we? The main reason that we do give plastic to children is that we assume they don't mind, and because we assume they will break things. Both sort of disrespectful assumptions really.
I like some aspects of the Montessori approach. Play schemas are a really helpful concept - that is the idea that small children are learning about the world through their play, and the different schemas represent different aspects (of usually physics) that they are discovering and testing out. For example, a child in the trajectory schema is interested in movement, so they will like to roll marbles down a slope, push trains up/down hills and throw things. Therefore, if you're having a problem with throwing behaviour, rather than just getting angry with the child you can see it as part of this schema and redirect to a less dangerous activity, like marble run, setting up a slope for cars, or setting up a safe/appropriate throwing activity.
In general I think the idea that children are small humans learning to be big humans is really great and I do love this, I think children are much more competent and aware than most people assume and give them credit for, and I think the Montessori approach generally gets this right. There's another fantastic theory that used to be quite fashionable (and OP would probably be equally irritated by!) called TCS - Taking Children Seriously.
I am a fan of anything that reduces my workload in parenting, so I do like some aspects of independence for DC and I have found in my own experience that for certain things, it does pay dividends if you let them do stuff themselves when they are still bad and slow and unskilled at it, because that means by the time they get to the stage of INSISTING that they do it "mine self" they actually have a chance of being able to manage it. Whereas if you always do something for them, because it's quicker, until they don't let you any more, they then try, can't do it and get unbelievably frustrated, which is just a nightmare for everyone. I'm not saying that practice at age 1-2 alleviates all of this, but it does seem to reduce a lot of it.
OTOH, I have plastic lidded cups/sports bottles for them to drink out of, even though I really hate drinking out of these myself, because I really dislike the extra work of clearing water up off the floor/table/everywhere else when they have decided that they are in the containment or transformation schema and need to experiment with how liquids and containers work 
Luckily we live in Germany where the early education is extremely influenced by Montessori type principles, so I have effectively outsourced them learning to drink from open cups, and they can both manage this well including the 2yo. It just takes more supervision than I can be bothered with to prevent the water experiments.