No, not really. If you run out of prepaid credit during the outage, your phone will still be able to make calls as normal, BUT: when you next top up after the outage, the value of all calls made during the outage will then be debited from the amount you are topping up by. So it's not free - it's just essentially delayed billing for that period.
You are assuming that the sim is ever going to be topped up again. Go for it and then come monday bin the sim and buy another.
That could be inconvenient as you would have to tell everyone your new cell number . Way easier to port the number..
How about data? Will data be charged at the casual rate? OUCH