I also really enjoyed playing tri-level when I was a good 3.5 player. Since become a crappy 4.0 player I have never been invited back. Definitely is an unofficial all-star league in my area. By all accounts I've heard, the National Invitational is a great event for those who make it there.
I also think it's a good sort of format that local leagues can use to get creative. For example, my area has experimented with different "18-39" leagues for younger players to have a league during 40+ season (which does not overlap with 18+ season here). Most of the 18-39 leagues in prior years failed to attract many players, but this year they tried making it an 18-39 tri-level league and it was more popular.
It could also be a good way to get 5.0 players into leagues where there are not enough of them for the traditional 18+ league. Why not make an "advanced tri-level" league that is 4.0/4.5/5.0? Would not need many 5.0 players to form a small league of teams.