Of course, since most layouts are evenly spaced you can always measure to find one that had it's pencil mark mudded over.
I've personally never happened into these flangeless cans but I've been thinking about it and I like the idea. when I use my rotozip to cut around the flanges there is little room for error, as the thickness of the bit riding on the outside of the flange makes for a hole just slightly smaller than the finish ring. Every time the rotozip decides to follow it's own path (and it sometimes does), I end up with a few cutouts that need a little extra attention when mudding the room. But if I'm cutting inside the perimeter of the can, the cutout is no larger than the inside of the can. Also, I don't thing the rotozip could take off on it's own if it wanted to.
I'm sure it doesn't make a difference to the client (I'm a contractor), as long as the lights are of a good quality and offer a reasonable selection of finish rings. But if the contractor is choosing the cans to be used, these seem like a really good, time saving choice.
I don't do anything electrical anymore, but I still like to know what excellent products are out there. Thanks for starting this topic.