I have multiple putters for practice and also to rotate so that they don't get beat in too quickly.
I have a bunch of different teebirds in varying plastics, weights and stages of wear. I can cover under to over stable with a combination of the previous variables. Having a stack for field practice is a plus and having backups is definitely good too.
I don't have many multiples of my mids because there is usually less of a chance that I will lose one.