I'll preface this by saying that I'm relatively new to the hobby (a little under two years) and I have had a hippo tang in my 75 gallon tank for about a year and a half. I understand where the "tang police" are coming from to a degree. I myself get irked when I see people with tanks that even I can tell are way over-stocked--or when I see someone with a small yellow tang in a 20 gallon or something--but I still manage to be polite in explaining why I don't think their systems are going to be successful with their current stock.
However, despite the large number of people who insist that I should have at least a six foot tank for my tang, there are at least a few with a decent amount of experience who will say it will be OK in a 75. Without any rigorous scientific studies on the matter, how are us non-experts supposed to know who to trust? I'm not one for simply siding with the majority on any issue.
I have no issue with people strongly suggesting that tangs should be kept only in larger tanks, but I see no reason to act as if this is some kind of commandment passed down directly from the reefkeeping gods along with "thou shalt not use silica sand" and "thou shalt quarantine thine new fish". The simple fact of the matter is that we are essentially using these creatures as ornaments. If we were really that concerned about their well-being, we'd leave them in the ocean. So any I find any BS about the fish's welfare to be just that--BS. Even in a six or eight foot tank, these fish will not reach the same sizes they would in the wild because of the sub-optimal living conditions. What we're really concerned about is whether it will in some way or another make our reefs ugly. In this case, that would happen as the result of the large amount of waste produced by such a large fish in a relatively small tank, or by the fish becoming diseased due to the stress caused by cramped quarters and dirty water.
Rather than spending their time going around insisting that tangs should be kept only in tanks of at least this length or that, I'd like to see the "tang police" go out and collect some data on the matter. Maybe they could conduct some kind of poll of hobbyists, and tabulate data regarding tangs kept in various size tanks, hobbyist experience level, and frequency if disease incidence in their tangs. I suspect that the results would support the positions held by the "tang police", but short of something like this they're still only working with anecdotal evidence.