Ah yes now your question makes more sense. I'm no expert so I'm sure someone will put us right, but the way I see it is the MCB is placed before the PSU because it's 3-4x (however many drivers you have) more likely to trip, so easy and inexpensive resetting is helpful. It's also very obvious when it's tripped rather than digging out fuses. There's also the chance of inrush to the torroidal tripping it, so again more likely. It's also a convenient way to isolate the PSU. As for the fuses on the drivers, they're cheaper than adding an MCB.