Is the valve being tested regularly? Manufacturers recommend, and many jurisdictions require that backflow prevention devices be tested by certified personel on a yearly basis. I usually recommend the replacement of backflow preventers when one of the following comes up... a device fails and the o.e.m. is out of business (Beeco), the device fails and it is more cost effective to replace it than to repair it, or if the customer is interested in replacing the valve anyway due to asthetic issues or increased volume demand. Remember, the valve is there for a reason, there are test cocks installed on it for a reason and if it is not being tested, how can you know if it is doing its job.
Hawaii licensed plumber pj-10296
Backflow prevention assembly tester no 12-00074 ABPA