Sometimes they come off. Most of the time they do not. I drove behind someone with clothespins on for hundreds of miles and never got one in my windshield. For the few cents they cost, is there a real concern? The clothespin issue is moot if you do not have spouts, is it not? So the same question slightly reworded: Are you trying to install clips or spouts?
The recipe for perpetual ignorance is:
Be satisfied with your opinions
and content with your knowledge.