It depends on what you plan on putting in QT. I qt everything. Inverts, fish, everything. If I'm buying out of a tank that is infested, I need to be wary of inverts as well. They probably aren't going to get sick but they might bring a parasite or illness in. If I treat a fish in a qt tank with live rock... The live rock is going to be destroyed and then it will be unusable for inverts. Most of your fish meds are toxic for the little critters. My qt tank has several pieces of PVC
for hiding places. They can be washed each use and stored away until I need my QT. I keep filter media in my sump to be ready for the qt tank and then I pull it out when necessary. I set up my 30 gallon QT with that filter media and 30 gallons from my DT during a PWC
and I'm ready to go. 4-6 weeks in QT is worth it for me to not have to tear down my DT and treat my fish for 8 weeks.