IMO
I'm assuming by now that everyone has read one of the many posts about Jin 'graduating' from KAT-TUN and them changing the name to かT-TUN. If not then here, have a link.
So here are my thoughts:
It doesn't actually bother me. I joined this fandom after Jin had come back from his study break and to me, he never really seemed to be a part of the group. At first I thought it was thanks to a bit of a diva attitude but the more I saw of him the more I noticed that he just wasn't comfortable there. Sure he has friends in the group but he obiviously doesn't fit in with all of them and it makes him uncomfortable.
This is the kind of job where you get given a position when you are a teenager and are expected to keep it for the rest of your professional life. Any other job would allow a person to change their job, either by going to a different company or just changing positions. But that doesn't happen in Johnny's. Even getting a promotion won't change their work. How many other people have walked away from a job because they were uncomfortable with the people you worked with or the atmosphere in the workplace? I know I have. Twice (out of four jobs).
From what I can tell Jin's management was aware of the problem for a while and they've been working to get him out of the band without ruining his career. It was probably thought to be an impossible task but Jin's too much of a cash cow for them to just let him go and he'd really struggle to get a career in any other area - being an idol doesn't make for a particularly impressive resume in any other field and I'm sure Kitagawa would have all his employees on an exclusive contract.
So I say, let him go. Let him salvage what he can and get on with his life. Honestly I like him better for walking away and letting the other guys get on with their band without having to hesitate or wonder about their future.
And in the end, I started following KAT-TUN and J-Pop in general, because of Taguchi's smile and then Maru's personality. Jin's style and his ever weakening voice thanks to an excess of cigarettes have guaranteed that he was never going to be my favourite so it doesn't really bother me if he's there or not. I hated that Jin was with them when he didn't really want to be and I agree with Kitagawa that he shouldn't have KAT-TUN to fall back on, if only because it's not fair to the other members.
Still, here's hoping that Jin thrives as a solo artist and it lowers his stress levels enough that he can quit smoking and really make a go of it.