Some thoughts about READMEs in the context of open source projects:
There's no need for the README to excite me and make me want to dive deeper into the software if there are good blog posts about actual use cases. They would lead me to your repo where I could find out how to install it, what the dependencies are, what kind of things can be accomplished in just a few lines of code, and how to contribute.
But, as a developer, I don't necessarily start a new project with exactly those points in mind -- and why should I? Most of the time, I intuitively start somewhere in the middle and move up and down from there. It is only after some time that I recognize what this thing actually is that I'm building. From my point of view, that would be the appropriate time to write a README.