I do this quite a bit. I was always taught never to allow the leader to disappear into the case to stop light leaking in (especially with well used hand rolled films). It also allows you to put the weight on the film before you go into the dark to open the canister. Just tear the leader if you have finished the film to stop you shooting it twice.
Only downside is loosing some exposures as others have said.
The entrance to the cassette has a velvet type fabric inner surface to stop light entering, but don't know what you mean by weight on the film before entering the darkroom. Can you please explain?