How is using semver casual? In AngularJS they introduced breaking changes a number of times, without a major version bump. Using semver makes it really clear, and since they are being so strict about it, there is greater confidence when updating to non-major releases. I think this is great.

Their 6 month release cycle is also very smart and allows for planning. I know that I'm only ever going to need to fix things once every 6 months when updating the library (if I do choose to update it) which seems pretty good to me.