Bottomline, If you keep a tang in a tank dubbed to small (less than 75 gallons) It may be "happy" for a year maybe two. After that it will not have enough roam to "race" in your tank. Then It will become stressed and probably show signs of ick (as I believe Ick is always present when fish are around as it seems to be a stress related issue from my experiences). The ich will most likely transfer to your other fish, causing you to treat for it if you don't have a tank wipeout and catch it in time. This will be headache as treatments are never pleasant fro either party. After your ich ordeal your tang will still be "unhappy" and cause another outbreak thus continuing the cycle till till your tang finally dies, if not the other inhabitants as well. Yeah, sounds like it is worth a shot to me. This sounds like so much fun everyone should experience this form of the hobby. (sarcasm off)
But people will do what they want to, because as humans we are natrurally selfish and what we want overrides all other concerns. So those expressing that the yellow tang can survbive happilly in a 55 gallon tank, you do that. And keep dumping money in a bottomless hole. Because after all, it is only money and fish you are throwing away. after you have a couple tank wipeouts like I have than maybe you will listen......Hopefully it won't take you $2000 dollars in fish and medicine related products to realize this as I did.
These people aren't saying not to do it because they don't like you or feel Tangs suck. They are telling you this because past experience and knowledge has proven it. So enjoy your tang, hopefully it will make it 2 years. The rest of us that own them in the proper conditions will enjoy ours for the next 10-20 years. How many tangs will you have purchased in that time frame to our single tang?