First, that expression is in the Official Languages Act. It is useful to have a kind of consistency from one act to the next. In addition, what I consider useful in this definition is that it is very general. It does not go into detail.
The nature of bilingualism is such that, according to specialists who have studied the matter, some people have a dominant language in one field, but another dominant language in another field. In the situation of our agents of Parliament, we should not be concerned about the ease with which an individual talks about sports in his or her second language, but rather the ease with which that person discusses his or her mandate as an agent of Parliament.