There don't seem to be any other "music"-based sites here on Stack Exchange, so why is this SE dedicated to "Musical Practice and Performance"? The one question I've asked on the main page has nothing to do with either of those, but it does have to do with music and seems to be on topic.
There have been other Area51 proposals about music appreciation and the like, but they never made it to beta. This site could probably also be called "Musicians", since that is the "Area of expertise" that we aim to foster in the community. General "Music" topics would probably include things like "How much coke did Van Halen do in the 80's?" or "Who's the artist on this album cover?" -- not exactly lining up with the musician expertise in the community.
In general, questions about rhythm are directly related to musical practice and performance, since rhythm is a fundamental practice of nearly all musical study.
To be honest, the question you asked isn't something I would consider to be a great on-topic example, since it's a pretty localized identification question. There was some good opportunity to talk about what is and isn't polyrhythm, but for the most part, I think it just piqued enough people's interests that it got a bunch of upvotes. Nothing wrong with that! The community defines the site, after all.
Speaking as a low-ranking but proud community member, I'm thankful that the site is confined in the way it is. The broad use-case may seem to be more inclusive, but it would very easily crowd out the current community with a very different center of gravity.
It's easier to see by comparison: the equivalent would be asking one of the technology-related SE sites to include questions about the latest apple fanboy rumors. The site would cease to provide a unique service since there are almost limitless places on the web for apple fans to muse just as there is a perhaps similar number of sites for music fans to gather and discuss minutiae quite apart from content that musicians themselves would find useful, gravitate toward, and contribute to. I don't know of a single place on the web quite like music.SE and I hope the community continues to agree that its fundamental character should reside somewhere in this neighborhood.
But I would agree that we need other music SE's so that questions can be diverted and given voice without putting off people who would probably otherwise want to also contribute to this site.
There are a range of other Music proposals in the pipeline, each appealing to different areas within the wider topic 'Music'. The two furthest along the 'definition' route are Music Fans and Music Production, but many don't fit with the SE model as they are very subjective.
Here, we try to focus on the answerable questions, which as @NReilingh said tends to lead to specific questions that musicians are likely to encounter.
If it were just "Music" then we would have much too broad of a subject to be able to address it adequately. Instead, Stack Exchange decided to focus this stack on helping musicians and music students in learning and performing music. That's a much more manageable topic.
Speaking as someone who is very active here, overly broad and vague questions are just not fun to answer.
Also, Stack Exchange, as I understand it, is geared toward helping people find specific solutions to specific problems. Stack Exchange is not a discussion forum per se. If we were to open this up to anything and everything involving "Music", then we would have a lot of posts that are essentially lengthy discussions and debates and sharing of opinions. Many forums exist for that sort of thing, but Stack Exchange is set up to be different.