I was just discussing with D3D some of the finer logistics involved.
If they occasionally have an indestructible urinal cake there - why must they ever take it away?
my theories are as such:
1) the dude in charge of changing the urinal cakes, he too gets bored and wants the ones that slowly disintegrate to come back. we all miss them when they are gone.
2) the indestructible ones are in fact just rented - you only get them for so long, and then they are collected and sent to other buildings, where employess there will look down with a sigh of disappointment as they see that their stream of urine has no effect on the structural integrity of said urinal cake.
3) the ones that don't break down will run out of smell, and at that time they are collected and sent back to the factory where they are carefully washed off and then reinjected with the lemony-freshness that we all have come to enjoy, then they are packed up and allowed to roam free again.