When you stick to a routine, you work up to using bigger weights. When you switch routines, you generally start off a bit lighter. So if you constantly switch routines then you are constantly using lighter weights than you are capable of using if you had stuck to it. You want to continue to use heavier and heavier weights, so in that case switching routines frequently is a bad thing.
If you like to switch routines a lot, then a good thing to do is stay consistant with a few of the core lifts and switch up the accessory lifts to suit your mood. For bodybuider tyes; for "Back Day" you could always start with a deadlift using a consistant progression scheme, but follow it with different upper back movements. For "Chest Day", start with a consistant bench press progression, then add in different chest and tricep movements.