Bringing down an entire application is easy. All it takes is the failure of a single service and the entire set of services that make up the application can come crashing down like a house of cards. Just one minor error from a non-critical service can be disastrous to the entire application.
There are, of course, many ways to prevent dependent services from failing. However, adding extra resiliency in non-critical services also adds complexity and cost, and sometimes it is not needed.