How many issues do you think may be hidden by mocking out the database layer? It's easy to write bad data due to a subtle bug in a query builder for example and then write garbage into the database. Mocking would make that significantly harder to catch.

You're quite right, query builder statements are error-prone and difficult to stub; that's why you shouldn't sprinkle them throughout your app. Rather than write things like

  User.active.in_group(x).not_replied.each { ... }

all over the place, write scopes on your User model (e.g. User.awaiting_response_for_group(x)) that represent each use case, test them thoroughly against the database, and then stub them out when testing client code. That way you only have to test the queries in one place, and you don't couple the rest of your code to your query builder.