This has been trialled at some sunday league levels for the past year or so, though unfortunately, the game does stop for each one. Probably good for a pub league player to have a breather, but if it was to ever be rolled out at the top level, subs should just happen quickly when the ball is out of play, and the onus is on the team making the sub to get players off and on the pitch quickly, and not have the game completely halt until they do.
I think that should be the case with all subs anyway, whether rolling or not. The whole 'walk slowly from the opposite touchline' tactic is the worst. Either stop the clock, or wait for a stoppage and get your subs on quickly.