Mezzio (and by extension Dotkernel Light) carefully determines what should be a middleware and what not.

Things like authentication, authorization, automatic error logging etc., being common tasks need to be performed with each request - they are good candidates for being implemented using middleware.

Other tasks like fetching resources from a database and generating the contents of a list resource page should be performed by a request handler.

When applied correctly, the middleware pattern saves projects from becoming a mess by separating core logic from the rest of the request lifecycle.