One objection in using MCB's as a conventional switch is that mechanically they aren't designed for the purpose, having a lesser number of on/off cycles as compared to a proper switch. A few years ago I was shown the insides of a Westinghouse Quicklag breaker used as a switch and observed that the copper braid to the moving contact wasn't in very good shape from the continuous flexing.
This particular breaker was being used as a main switch for a laboratory; rather than turn off all the equipment individually at the end of the day, the main circuit breaker was just used to turn off the whole lot instead.
Having said that, other MCB's might fare better. The problem was solved simply by fitting a 45A 3phase switch at the bottom of the breaker panel for the staff to use instead.
