It lead to a lot of design problems, mostly because standard brass cases would draw heat out of the whole system, and without the case there the barrel heats up a lot more. Combine a hot barrel with exposed propellant and you've got problems on your hands.
It was popularised in Call of Duty: Black Ops, along with the Enfield EM-2 prototype, of which less than 20 were produced. They're both just experiments, and the G11, despite being a competetive rifle, was just an expensive toy.