How long would a vehicle be allowed to sit in the workshop? If the engine is out and is in pieces, the owner is hardly going to want to have to have it transported back and forth from the workshop to fit in with a roster. If they leave it in the workshop it is going to take up space, if one person is allowed to leave their car in pieces there then everyone else is going to expect to be able to do it.
What happens when a part of theirs goes missing? Even the closest of friendships fall apart when trust becomes an issue.