I think the distinction is whether they operate as cross sensors. With 11 point, it's easy to end up with an AF sensor that is not a cross sensor. While that's okay for f/5.6 and lower, at greater than f/5.6 it can make a difference in AF performance and cause you to miss focus.
This is even more of an issue with an f/4 lens and teleconverters. If you use a 1.7 or 2.0 teleconverter, its easy to end up with an AF sensor that does not perform as well as the center sensor. The same issue applies to a variable aperture zoom. In both cases you need to be aware of the impact on AF from the narrower aperture - not just whether AF occurs but the accuracy of AF as well.