I think boxing is pretty valuable on many levels for becoming a fighter. It teaches how to roll with getting hit, forming a strategy, and handling distance. Just my opinion.
I started aikido in '91ish, and found that to be valuable. Around '93, the school I was going to started co-op training judo with another school that has a great history (they taught us judo and we worked striking with them). I had more formal j-j training some Gracie affiliates starting, eh, mid-90's. All of it is valuable, and I learned something from everything.
A bunch of the people we did j-j and striking with went "grappling exclusive" when the Gracies were dominating everything. They did very well with regards to competitions and teaching, but most either slowed down or dropped out completely due to injuries. I think one guy still does j-j only, but he's a physically huge . . . knucklehead. Granted, he is a very talented grappler, but he likes it a bit too much. He especially likes grappling with smaller people that he can dominate. Eh, he's a big, strong, reasonably trained bully, and he's injured a few younger students. They're still young enough to buy the old "oh, pain is weakness leaving the body" crap. He told me that once, and I told him to let me set his balls on fire and see if he becomes the incredible hulk. It took him a while to process that, I think.
With regards to MMA, it's almost required learning for any fighter i.e. grappling and striking on the ground. There are some down sides which are occasional tunnel vision i.e. "I must fight THIS ONE opponent, and I have plenty of time to do it". It's great for the times when you have one guy AND ONLY ONE GUY to worry about, and you have all the time in the world to deal with them. I find that what most people call MMA is severely lacking in the old "there's TWO OF THEM" techniques. The last place you want to be is on the ground holding onto one guy while his friend is pounding you with a stick. I assure you that my j-j will surpass any world class fighter IF, while we're on the ground grappling, one of my friends is kicking him in the head. My j-j is VERY good on a semi-concious and bleeding from the ears opponent.
Another down side is the injury factor from MMA training. Most all of the guys who went j-j / MMA several years ago have got scars, and I mean from back, shoulder, elbow, and knee surgery. And many of these guys are early 30's, and their injuries are lifelong to varying degrees.
While lots of you guys are young, and the thought of permanent injury isn't really in the front of your mind, you might want to ease it forward. MMA is REALLY hard on the joints, and the human body isn't really designed to spend decades in heavy practice and competition in "all out" fighting. Sure, there are a few who can, but I'm talking about on a mass scale.
I'm only 41 and I limit my groundwork SIGNIFICANTLY. My life can't tolerate a "you'll be laid up a few weeks before the surgery and then 6-12 weeks after" injury. Sure, there's danger in any sort of contact training, but MMA / j-j, in my opinion, has a much enhanced potential.
Good stuff, though! Any martial artist should train it as much as they can comfortably.