It isn't a bad idea except that you would have to keep it provisioned and ready to sail - then you would have to do ship to ship transfer of guests and crew which is really considered pretty dangerous.
I think in this case Carnival was really hoping the ship could get towed in faster. It is had been over in 2 days instead of five this would have been a completely different situation.
So - maybe what they NEED is a majorly big TUGBOAT ready to go out and get any ship and tow it in as quickly as possible to the safest port.
two times now a ship dead in the water has drifted overnight adding difficulty to the tugs reaching her.
One big tug in the Western Carib, (Grand Cayman) and one in the upper southern Caribbean St. Martin, could conceivably get to any cruise ship quickly.