Why would someone choose to fly VFR on top?

Bob Gardner wrote:

Example: I know (or strongly suspect) that the tops at at 6000. I call
Ground and tell them that I want an IFR clearance to VFR-on-top. No flight
plan filed. Ground says "Whizbang 1234X is cleared to (nearby VOR), climb
and maintain 7000, if not on top at 7000 advise. Squawk 3456." Great time
saver.


What happens if there's a comm failure and you're wrong about the tops? Of
course, this is a general question about any clearance limit that's not an
airport or certainly VFR.

- Andrew