No water allowed !

Like in most of the gyms, we warm up, sharpen some technique, drill a bit, and finally we roll. Our coach allows us to drink right before the rolls begin, but inbetween rounds (we roll 7x5min on average), we're not allowed to drink. He says that it's to train our resilience/cardio, but is probably just a way to avoid time wasting when switching partners. Is he right or is he telling bullshit ?