I think this is the situation in a nutshell. The most difficult outcome (besides multiple deaths) is @ErikVR 's situation where he has a large display tank and a disease outbreak. I think we (the Fish Medics) discuss these types of cases at least 2-3 times/day.A good thing is that many of the fish we purchase from reputable sources probably are not carriers of disease in the first place. When someone successfully adds a new fish to their tank without medicated QT, the likelihood is that the fish was not a carrier of the parasite in the first place. The tank doesn't have the ability to immediately to transfer some magical immunity to each fish added.
What are the odds any individual fish is a carrier? Way too many variables to be able to make a reasonable guess. Could be 0% if the LFS or Mail Order Vendor follows very rigid protocols or 100% if the fish was visibly infected when purchased.
Over the past several years I have purchased a number of fish from several different Mail Order Vendors. I have followed the medicated protocol recommended on this forum. Over that period, I have not had a case of either ich or velvet. Fish that expired while in quarantine died from obvious starvation or bullying. Fish that survived were successfully moved to the display tank.
Does that mean my QT process eliminated ich and velvet? I have no way of knowing, since the fish may not have been infected in the first place. Likewise, every fish I acquired may have been a carrier and the protocols vetted on this website did what was intended.
If I had placed the fish directly into the DT, and later determined the fish had parasites and "needed treatment", I would have faced a nightmare. To take care of the infected fish, to take care of the established tank population that had been exposed, and to keep my tank fallow without harm to my coral would have been a true headache with no guarantee for success.
Like everyone, I want to get new fish into the DT as quickly as possible. For me, the risk of failure and the repercussions does not justify shortcuts. Everyone has to decide for themselves what risk factor is acceptable.
Most of the time people have these issues is similar to many of the anecdotes here - I had my fish in the tank for months, never QT'd, feeding well, etc - but I added a 'name that tang' and now many are sick, dying or crashing. Many of the more vocal proponents of QT are those that have had this happen to them - or have read the many many posts from people who have had this happen to them. People have mentioned another reefer who has had a tank for 40+years as an example of 'ich management' however, they ignore the 1000's of new reefers that read those posts and think - oh I can just manage Ich, and subsequently have dead fish.
I think after multiple discussions about this issue with @Lasse, @atoll, @Subsea, and @Paul B that indeed having a tank full of inverts that perhaps eat some parasites, along with a relatively low stocking density can indeed 'work'. I also think a UV would help if it can be maintained and is large enough. If I was forced to start up a new tank - I would use @Lasse 's method - hopefully he will send the link- as I can't find it. The reason - I have years and years of experience and know how to treat/diagnoses disease and treat it if needed. Many new reefers do not.
I do want to repeat a point made earlier. If you have a tank using ich management with presumed parasites and start adding tank-raised fish you may be at higher likelihood for disease, since said fish has no natural immunity.