The main advantage of server-side rendering of client-side templates is cutting your initial load time to a bare minimum (not having to wait for scripts to load/compile, taking advantage of page caching, etc.). However, this usually only matters when you're afraid of users getting impatient and bouncing, which tends to be true of sites that mainly display content and don't have a ton of interactivity, and sites like that usually shouldn't be using client-side rendering in the first place.

So you have two buckets: web apps that don't care too much about shaving milliseconds from the initial load, and content sites that shouldn't be shipping heavy JavaScript in the first place. The target market for hybrid rendering is the tiny slice of sites in-between those two, so it's a minority of use-cases. Though it is also used by sites in the latter category that are shipping heavy JavaScript anyway and having regrets.