Think about it this way... The meds we use to treat fish are "antibacterial" , anti-parasitic, etc. If you treat a tank with bacteria covered live rock with a med to kill a bacterial, fungal, parasitic infection on your fish.... The med isn't only going to kill whatever is hurting your fish. It's going to take out the stuff in your tank as well. That's why you can't treat your DT. If you have an extra amount of live rock lying around that you don't ever plan on putting back into water with inverts or other live rock... By all means use it. You may not have to treat the QT tank. After all QT's are precautionary, you don't know for sure
If your new fish are sick. But if you're putting the LR
back into your tank after you QT your fish.. You're missing the point of quarantine. If you plan on leaving the QT tank running forever, then that's different. It's only ruined if the fish are sick and need to be treated. I don't use live rock in my QT/ hospital tank.