Most folks working in docs-ui rely on the ability to test PRs against stable services which match production. These include content services like OPS, docfx, DHS, content-proxy, and learn hierarchy, but also profile, progress, gamification, and others.
For devs working on this type of functionality it would be detrimental to reliability of the product if they were integrating PRs with services which do not match production. In the past there have also been productivity issues when we were developing against PPE versions of services. For example, during learn's early development we had to use the PPE environment's nightly builds of OPS and docfx. When there were bugs in these services the US team was essentially down for the day because we couldn't integrate. With the current team sizes the impact of a down service like auth or content would be significant.
Another set of folks working in docs-ui rely on the ability to test PRs against unstable services which may n