The original pull request includes some metrics on the gains: https://github.com/alphagov/whitehall/pull/186

So a counter argument to including those assertions is that it will make the test more brittle in that it's more likely to break when the view is tweaked. For example, it might be decided that it's not important for the user to see an announcement's topic on the index page so it is removed from the view, which would break the test. Asserting that the topic is present is putting a hard requirement into the codebase that the topic must be listed on this page, which isn't necessarily the case. That's why I tend to only assert the critical information is present as the rest is subject to change. This keeps the code easier to change.