For me "A must" = Yes. I just have a 10 gallon with HOB, heater, Par38 bulb and a piece of PVC for shelter. I have been in this hobby 8+ years and can confirm that I have had ich, flatworms, bubble algae & mysterious deaths all in my QT tank. Is any of that in my DT or ever been there? No.
There are enough other thing that can/will go wrong in this hobby that you really want to minimize the risks as much as possible. Sure the $100 QT seems like a lot, yes it is no fun to QT, takes time & space but, one pest can really screw up your DT for months and take tons more time and money to fix than a QT ever will.