When I go on vacation I worry a bit, even knowing I have a back up. I could not imagine leaving for a few days without a back up drain. I would suggest adding a top to the overflow box if you dont already have one.
If anything gets stuck in your drain like fish that jumped over, seaweed, snail or a combination of anything or even build up in the drain or you accidently adjust your drain gate value incorrectly and your tank water slowly rises, there is nothing to stop water from going all over the floor. My back up drain has been needed a couple times over the last year :) On the other hand many have kept tank with one drain for many years. Better safe than sorry though.
