T o d d P a t t i s t wrote:
No, it's not very common. It's entirely legal, however. If
there is no intent to take off, there is no requirement to
have a pilot's license of any kind.

Questions of intent (to take-off or otherwise) are an open
invitation for a litigation problems involving cuplability
if some accident or incident ocurred. It won;t really matter
if what happened was "legal' or not, if someone sues its up
to a judge or jury to decide.

The FARs state that you can log time (PIC or otherwise) whenever
you are the sole manipulator of the controls and the airplane
is moving under its own power. Personally, I would never allow
a student to go out an operate an airplane alone (taxiing or
otherwise until he was signed off for solo. I wouldn't want to
have to explain such a situation in court if anything happened.