They are right - MLV vaccines need to be used within 3 hours to be effective. You are relying on the virus to be alive in order to replicate and start the immunization process. There is not enough virus in a "dead" MLV to get the same response as a killed virus vaccine,so once the virus dies it's absolutely no good.
Cattlemaster is a MIXED MLV and killed virus vaccine, so the same thing applies. After 3-4 hours, pitch it.
Be careful with keeping opened killed virus vaccines too long - they can get colonized with bacteria and fungus. They make special heads you can put on the vials to prevent contamination (cost ~$2) but they are a pain in the arse to use as you need to remove the needle to withdraw every time. If you are vaccinating breeding animals you should change needles between animals anyway to prevent spread of diseases such as leukosis anyway.
Yes, it sucks to throw away unused vaccines, but you may as well be injecting water for all the good they will do you if you keep them.