MR board is cheap insurance for any area that will see higher than normal levels of moisture. Bathrooms, laundry rooms, even behind kitchen sinks and dishwahers. Proper ventilation is spotty in most cases. It may be perfect at the ceiling level. Effective half way down the walls and non-existent behind that sweaty toilet. Any accidental spills or overflows that reach the bottom of the wall board aren't going to wick up into MR board like they will regular drywall.
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