My 2 cents...
D2 has been a great addition for L5. Lots of teams using it here, and really helps the newer L5s feel like they can have some success and stay motivated. Most L5 boys are still fairly young and new to gymnastics, and getting crushed by 3rd year L5s with all bonuses in your first year (often at a lower hours gym) can falsely signal to a boy that he isn't 'good enough' just when he is getting started. L5 is a key foundational set of skills, and the first 'real' level (imo - as nearly any kid can manage L4, but L5 starts to require work), which is why many gyms repeat it so much. This results in a ton of variance between kids who are 'beginning' L5, and 'mastering' L5. D2 makes sense here if keeping boys in the sport is the goal.
D2 for L4 is fun and cute and lets the littles get out and compete earlier with less pressure from their gym to get them a ton of bonuses and be "competitive". I think that's a good thing. I don't see it as so beneficial as the L5 D2 separation, but I saw a ton of cute little guys new to gymnastics performing their little L4 D2 hearts out and proud parents snapping the cutest photos ever. So I'm a fan.
D2 for L6 and L7 is where the division starts to lose me. Just don't seem to be enough kids. Especially L7. If you are advanced enough to be doing L7 skills and have stuck around this long, you know the game and you know where you are. I'm not sure L7 D2 adds a whole lot. There are some gyms using D2 for L6 and L7 here. So far, the numbers are small. I do personally know D2 L6s who are happy the division exists and still beam getting a medal or 2 for their efforts when they otherwise wouldn't have. I know we over-medal here in the US as a trend, but medals are cheap and if more divisions are what keep kids motivated to stay in (because it helps them feel like they are 'in' the game, as opposed to 'out' of it for their age/level), then I can endure the longer award ceremonies to support that.
And JD rocks! Perfect for so many boys who don't 'fit the mold' of L8-10 (yet or ever), but don't want to be stuck to a formula in compulsories that isn't optimized for their strengths and progression plan. Also great team camaraderie for boys in this category (all basically teens) to have their own 'tribe' so to speak. LOTS of JDs here. Big fan of this new division for many reasons.