Who said it isn't a go to for a DT? Every tank i have had was started with Dr. Tims. As far with QT, there are situations where the fish needs QT right away and if one does not have a permanent QT tank the live bacteria is the best way to make the QT safe so the fish can be treated asap.
I personally think it a great way to cycle a tank. You are introducing bacteria that would take a lot longer to acquire.
Of course there are boundaries, i would not add the bacteria then fully stock the tank with fish within a couple of weeks. I would do this over the next couple of months.