As much as I hate the whole quarantining process, I think that it’s a solid foundation to a successful population of fish. Now, disclaimer, you don’t need a qurantine system, and a lot of veterans in the hobby will tell you ‘do as I say not as I do’, as many don’t actually quarentine fish because they have a great relationship with their suppliers and a nack for identifying healthy specimens.
I also think a lot of people put off the whole ordeal because they think it’s expensive and you need to have another tank setup. I have my old 20 gallon and a hang on back filter as my QT system and I only set it up when I get new fish. I have a huge roll of filter floss that I cut into squares to fit inside the HOB filter and I always have a nice mature filter pad in my sump. When I get a new fish I fill up the QT with display water and pop in the mature pad into the HOB filter and a fresh new pad goes back in the sump. When the new fish is a good to go, the old filter pad get tossed, tank is taken down, rinsed, put in a large garbage bag, and outside it goes. This minimizes any ammonia spikes, gets the fish into nice and mature water, and makes the process easy and streamlined—thusly, I never needed to have a QT system live and on standby.
Now, of course you can get nasties on corals, snails, and inverts (I don’t quarantine them), but it’s the fish that generally carry things like marine velvet that can wipe out your prized family members. So in the end it’s ultimately your choice, but as with everything in this hobby, I would recommend you lean on the side of caution and have good husbandry skills—remember, an ounce of preventaion is worth a pound of cure.