I used to feel this way, not for things like "validation on assignment" which is a terrible idea for so many reasons, but because it allows you to build things

I used to feel this way, not for things like "validation on assignment" which is a terrible idea for so many reasons, but because it allows you to build things like dependency graphs and change watchers at runtime (think mobx, or Adobe Flex's bindables system)... But these days, I think we'd be much better suited to moving most of this work to build-time tooling.