The paper "Unified Call Concerns" (linked at the end of the post) describes the issue in a bit more detail: you might write f(X x, Y y), knowing that X doesn't have a member function f, and call it with x.f(y). If a new version of the library extends X's API to include the member f(Y y), your code will suddenly start calling the member instead of the original non-member function.

As he mentions in that paper, this isn't likely to be a common issue, so I'm with the general consensus that it seems unfortunate that the feature wasn't adopted.