You are assuming that all apps are generic. Some apps actually are specific to a medical condition you can have or certain aspect of your life that are not known by the general public. It is of the same nature as your browsing history in some ways. So yes apps usage patterns are actually potential privacy issues. Admittedly since most apps are downloaded from the app stores Google would already know part of the story but still your usage pattern may reveal even more especially when correlated to other data.