I can't say I've seen it very often, but here's 2 possible reasons why stopping the engine just before touch-down might be a good idea:-

1/ Pattern aircraft are as streamlined as possible to reduce drag when performing smooth slow aerobatics. This also has the effect of making it difficult to stop once landed, especially with the engine running, even if it's only idling. Stopping the engine shortens the run-out.

2. Since some aerobatics demand slowing the engine to idle (hammerheads, for instance) there is a high possibility that the engine might stop. Increasing the idle a bit will tend to avoid this, but item 1/ above then becomes even more difficult.

My 2d!

David C.