If Fiverr allowed you to do gigs for free, would you? Basically, you would do a custom order and charge 0 dollars.
When things are slow, there are times I think I would do it if it helped my fiverr ranking. Basically I would do it for free if the client help me spread the word about my gig.
No doubt, I would be very selective and may only do it a few times a month but it may be worth it if it helps me get more gigs.