I guess times have changed...
I have a box full of these things, including a few ancient bakelite ones sitting bagged in the freezer that came to me with good film in them(some old Velvia, etc). The old Bakelite ones I have look/work just like the Watson style, but are smaller so waste less film in daylight.
I don't think I've ever bought a 35mm one-I got my first one in 2006ish when I bought a camera(Canon T70) on Ebay and the seller asked if I wanted it. All the others along the way have been throw-ins when I was buying other stuff. At the camera store where I use to hang out all the time(the legendary Chuck Rubin's in Louisville, KY), Chuck use to give me any that came in that had film in them-or I should say he might have charged me a few bucks, but they'd usually come in with other darkroom odds and ends that I wanted and he'd throw them in. That was especially true if they didn't say what they had. I'd take them home, snip and develop to find out what was in them, and then label. More than once I film tested a camera for Chuck with film from a bulk loader he'd given me, so it all worked out. Most unmarked ones would have some sort of B&W film in them-if it was a film I was especially fond of(Plus-X, Pan-X, etc) I'd keep it for my own use, but if it was Tri-X or whatever else with too much base fog to want for serious use but little enough to still give an image, I always considered it prime stock for basic camera function testing(light tighness, frame spacing, major shutter issues, etc).