Agile is a management tool. It helps manage developer time, feature bloat, organizational blocks, communication overheads and user expectations. It also helps you measure performance and identify bottlenecks in your own process. Like other comment said, it helps getting a couple things done instead of having a huge pile of stuff half done.
Look at what Agile is promising to fix. The development process not the management process. Semantics aside, I like everything you said, except to say that either the line manager is doing those things, or 'it' (scrum) is doing those things.
Managing the things I mentioned fixes a lot of problems common to broken development processes. [Insert your agile method here] provides a useful framework upon which to build a working development process.
