While it's cool and somewhat self-indulgent to say I spent 9 years getting 2 black belts in Karate, in reality it's kind of like having a PhD. It looks really good on the wall and it's a great conversation starter, but it doesn't really mean much on the streets. I have grown much more in training in different arts for shorter timeframes (say, 2 to 6 years). Why? Because no martial art is perfect. Despite what just about every practitioner of every style you have ever known will tell you, each one has core weaknesses that prevent it from being a complete self-defense system, or a complete physical training system (for those who swear they don't train to learn how to kick ass). Several have more weaknesses than strengths. For those who like to keep a balance sheet, these may seem like a complete waste of time. This is actually not so. For example (personal views here), Aikido has far more weaknesses than strengths. It is far from a complete system. A black belt in Aikido would get the crap pummeled out of him by an aggressive boxing student with 9 months of experience or a wiry wrestler with a few seasons under his belt. But, when Aikido skills are aggregated with skills of other arts, positive things happen. Even if all it does for you is teach to to take a mean break fall or apply a nasty wristlock (a technique of pretty limited utility on the streets against a wiggly opponent -- ask any cop), it still is better than not knowing it. But these arts (there are many that fall into this category) should be neither ignored nor focused on with too much intensity. Go in with a skeptical viewpoint, and like Bruce Lee said, take what you can use, discard what is useless to you.
I see these weaknesses as holes, like in Swiss cheese. Where one art has a hole, another one has a perfect plug for it (yet has its own holes elsewhere). The trick is to know where the holes are and know what other arts may fill them. Don't waste time if you discover an art that doesn't fill a hole you have.
Mixed Martial Arts (MMA) has arisen in popularity as more and more coaches and competitors come to grips with the reality of this concept.
We have a weekly MMA class to round out the skills of our students. I encourage all of my students to cross-train. They should get out and train with students from other schools as often as they can, identify holes, and plug them with specific training.