Tai is a commonname containing a variety of different fishes in the Pagellus catagory. The English commonname is (red), (blackspot) sea bream. I think this is not a very prominent fish in the US since I don’t see them being advertised a lot in Japanese restaurants here.
Probably one of the reason it did not get as popular in US is because of its inflexibility in the use of the fish as a cooking ingredient. It is a shiromi (white meat fish) and is mild in taste. Since sashimi in US has to be flash frozen before consumption, the taste would just be quite undesirable to consume as a nigiri sushi or sashimi. It would not be a good ingredient to make a chef’s creation role either. The texture is quite chunky, and since it is not a fatty fish, you can’t really bring in extra flavour in a fish. The next best thing besides making it in nigiri sushi might be grilling it with a sweet sauce.
Probably you should eat this fish either very fresh or don’t bother to eat it at all. It’s hard to find a good one in sashimi grade.
The reason Tai is not command is beasuse most restaurant serve Blackspot seabream,tilapia or other “cheap” stuff as Tai and serve the real thing red seabream as “Madai” at almost twice the cost. But on the sushi order guide, the translation for Tai and Madai are the same, most people will just go head order the cheap one, which have a complete different favor.
And I have to disagree with you on the “not a good ingredient” part, there are so many dish using Tai in Japan, some of which are favous throught out the world. If you can get you hand on Akashi Tai, the taste is out of this world, but at the same time it going to cost you almost $50/lb retail