One could say, that if you leave your lock on said rack, and a would-be thief notices said lock - and notices that the same bike is locked to it every day, could that not give said would-be thief all the time in the world to tamper with the integrity of said lock until one day they are able to just whack it with a hammer or some other simple final blow and ride away with said bike?
(I like the word "said")
*DISCLAIMER* I hardly ever lock my bike, maybe 3 times in the last year.