I started bulk loading some time in the early 70's and have been doing it on and off since.
Back in High School (late 70's) I shot high school sports for a local paper. They supplied bulk rolls of Tri-X for our use.
Memories of what must have been a very shy lab tech...
I'd ring a bell at the back ally entrance to the papers offices and the tech would open the door just a crack and ask "how many rolls?" I'd tell him how many I wanted and the door would close. A few minutes later the door opens a crack and a hand shoots out with a zip lock bag with my requested film. Later that night, after the game, I ring the bell and the door opens a crack. Give him my name and the hand comes out to take the bag, and there better be the same number of rolls as he gave me. It always felt like we were doing a drug deal, or I was a spy passing microfilm to my handler. I never saw a processed roll, just the small shot that would (maybe) be in the paper the next day. Too bad too, I wish I had some of that film now. John Elway was the star quarterback at my high school.
I've always used metal snap together cans and I don't remember ever loosing a roll due to one popping open. Must have happened at some point, but I have no memory of it.
Blockend mentioned the newbie mistake of forgetting to open the light trap before cranking the loader. Well, it's not just a newbie mistake but fortunately these days with Photoshop if you want to take the time you can remove the scratches. Lots of scratches though so pay attention, its amazing just how easily the film will run through that closed light trap...
I like to bulk load, and got started in it way back when, for the cost savings. That and the ability to load custom lengths. My shooting needs have changed. Back in the day we loaded long rolls, crammed as much in a cassette as we could without causing scratching. Now I load short rolls of 10-15 frames so I can get the roll done and processed. I just don't shoot as much as I used to, and have too many cameras loaded with film. Another benefit is the ability of having so much film in a smaller storage space.
Right now my bulk inventory is about 125ft or so of Delta 100, just under 100ft of Pan F, 100ft of Velvia 100, and 300ft of Ektachrome. I have two loaders at the moment but am about to pick up at least one more. I keep one loaded with Delta 100, the other with Pan F and I'd like to load one with 400 speed something (probably HP5 as Tri-X is stupidly expensive).
I cant be sure as I don't have any of the old reloadable cassettes around but it seems to me that the ones we can buy now are much lower quality. They just don't seem as secure.