I use quarantine tanks (Q/T) for all my new fish. It's not just the money, but I get attached to my fish. I know their particular personalities and quirks, and I don't want to lose them.
Regardless of how wonderful the LFS may be, unless they are breeding the fish themselves, they have no way of knowing the condition of the fish coming in. Many of these fish have been breed in massive numbers overseas where quantity is more important that quality. Fish diseases can be rampant in these places.
Then the fish goes through the stress of shipping and the store's different water parameters. Unless the store is going to do their own Q/T for 2-4 weeks (too costly). then what you are buying is not their fish, but the fish they received from their importer. Some stores make matters worse, no doubt. But no store can be certain all the fish are healthy when they just arrived in a box from overseas.
I quarantine my new fish because I do not want to introduce an illness into my main tank. I also think it's good for the new fish to have their own peaceful tank before they're thrown into my main tank with various schools of fish, dwarf cichlids and shrimp. Many of the new fish have never been mixed in with other fish, that has to be stressful to them. By allowing them their own Q/T where I can observe them very closely, ensure they are eating well, and if the fish do turn out to have a disease, it is so much easier to treat them in a Q/T than in the main tank. It's much easier to treat just one species of fish in a Q/T than to find meds that work for all kinds of fish, inverts and plants.
I do not use water from my main tank. I always use very fresh and clean water (conditioned tap water). I run the tank at least a day before adding the new fish. Depending on the particular fish, I may add caves, live plants (great for new shrimp), silk plants (unharmed by meds), floating plants or whatever matches the fish's preferred environment. I then watch the for 2-4 weeks. I do frequently water changes to keep the Q/T very clean, and I watch the ammonia closely.
After the fish have successfully completed their time in the Q/T, they are accustomed to me, used to the food I feed, acclimated to my water, and much stronger and less stressed than they were when I bought them. So moving them into my main tank is one more step for the fish, but it's not anywhere near as stressful as what they go through when you buy them. That little bit of stress usually doesn't harm healthy fish.
The day you experience or even read someone else's experience of having a tank full of fish they love, only to lose them all, one by one, after introducing a sick fish that hadn't been quarantined, you'll be glad you use a Q/T. It's heartbreaking what these people go through. I don't want to experience that with my tanks. So I use a Q/T for all new fish.