Ignoring even a small crack in a taillight lens can cause plenty of trouble for you in the future. Besides the obvious problem of perhaps receiving a ticket for the broken lens, a less obvious problem is water damage. Water can creep into the smallest opening, and if left unchecked, it can fog your taillight assembly interior and eventually short the taillight out. You can repair taillight lenses for much less money than buying a new assembly from the auto parts store, and it only takes minutes to do.
How to Glue a Taillight Lens: 7 Steps (with Pictures)