I have a
10+5.5 gallon combo coral QT/frag tank. I intended to manage parameters in it by just doing water changes from my display into the frag tank, ala what others suggested here. Practically speaking, I think it's kind of difficult to pull off, unless you fully automate it or are really diligent about it.
If I'm going through the trouble of pulling out my water siphon and a bucket, I'm going to want to do a bit of cleaning while I'm in the main tank. Maybe suction from the sand, or near the rocks, or whatever. I also am not going to keep water buckets in my living area, or at least I'll pretend I don't because my wife realllly hates when I do.
So for me it's just more practical to do water changes of freshly made salt because everything's in the garage anyway.
However, even there, eventually I found it was just not worth the hassle. It requires money to be spent, but it sure is convenient to just dose stuff into the frag tank. I'm currently just dosing All For Reef. Super easy. Just requires one dosing pump.
I also have two damsels in my system. One in the sump area, one in the main display. Goal was to have a nutrient source, and also the Springer damsel supposedly eats flatworms, and just looks pretty. In retrospect I wish I'd of just put a sixline in there. I can't see a situation where I'd regret having more bug control in the tank. That does mean the tank isn't fishless, so it can have a fish disease carrier, but that's a risk I'm willing to take for that tank.
Finally, one more side point, keep in mind if you're trying to be truly rigorous about those quarantine timers, those timers reset every single time you add something to that QT tank. If you're doing a 72 day QT, that means you can only buy coral / snails / ... 5 times a year. IMO that's just not realistic. I'm not quite at H2OPlayar levels of comfort of just having the tank solve it itself through wrasses and such, and I'm also dealing with monti nudis in my display right now, but I just can't imagine actually pulling off a 72 day QT.