As long as the luggage is buckled up in its own seat, I don't see what the problem is either. After all, most luggage is not nearly as big and round and heavy as most of the passengers, who are similarly buckled in. Even on US airlines (which are as anal-retentive as they come), cellos are allowed in the window seat of many aircraft.

Seems a heck of a lot safer than the abomination of practices: "lap children" (i.e. free-to-catapult babies).