One way is that, by having a "one holy solution", you avoid fragmentation and get more stuff working on end-user's systems with minimal hassle. This is why, for example, Steam targets Ubuntu. They're the 800lb gorilla of the Linux world, and if someone is a desktop Linux user, odds are high they're using Ubuntu rather than anything else. They target the most users with the least hassle.
The end user doesn't want to mess with dependencies, compilation, desktop environments, and so forth. They want their shit to work. Fragmentation is an impediment to this from a usability standpoint, and that's before we even talk about security (e.g. Android).
The other way is that by these tightly integrated, non-interoperable stacks like Unity and Systemd make life miserable for every other Linux user in the world who values the ability that they can sample this init system, that window manager, etc, and build a system that fits their needs much more closely than any of the tightly integrated one-size-fits-all solutions.
This isn't a false dichotomy either - there are no companies out there nowadays that make good software that works really well for end users with minimal hassle, while allowing the degree of customization and interchangeability that power users demand.
So pick your poison, I guess.
Most complaints about Canonical's exclusivity underpin a utopian fantasy that everyone in the Linux community must converge towards one holy solution. Any deviation is considered "fragmentation" and brutally discouraged.