I can see the benefit of a YOT build .being able to sling the seat under to lower the c of g. But what benefit from a standard Y against T frame,I can see that you can get the seat marginally lower but not by much before hitting obstructions, with T frame You can still get the seat and you,in line with the rear wheel centers . So any other advantage,that makes it worth the slightly more complicated construction to form Y and line wheels up.