In the narrow perspective of autolanding, altitude is fine. But does it make sense to narrow our sensor usefulness to only the current case? Maybe, although it's so cheap to get the other two dimensions I don't see any particular reason to limit ourselves.

* For quadcopters, sonar would be very nice in altitude hold while flying around. The hovering hypothesis will oftentimes be violated in these cases. 
* For fixed-wing, using sonar for altitude will by necessity require changing angles. This becomes particularly noticeable on landing if a sudden gust of wind causes the plane to roll.

Note, these aren't reasons why *you* should do the changes, just reasons why we would/should want to do it.