contra tcaud, it's not going to solve the world's problems, nor is it going to revolutionize we relate to one another. Even Freud didn't really do that, and his ideas were, frankly, better and defended with better rhetoric.
contra most people on this forum (Ashton, with a degree of facetiousness I can never determine), it's not useless either.
Socionics matters because people are different. Sometimes so different that they have trouble communicating. Have you ever been in an argument and you suddenly hit this point where it's like, "I'm confused. This doesn't even make sense. I know that what this other person is saying isn't totally wrong, but I just can't wrap my head around it, let alone agree with it"? That's socionics in action.
Socionics is for those moments in which nobody is wrong and everybody is equally right. Socionics is for those moments when what works for me doesn't work for you, and I have to step aside gracefully, understanding that I don't understand. Socionics is for those moments when you feel that divide that you're never quite going to cross between someone else's perspective and your own.
Now, does that mean that the "lesson" of socionics is, "stick to your own kind," except kind is a poorly defined grouping of poorly defined types with poorly defined information elements undergirding them? Not at all! Rather, socionics is a tool that you can use to navigate the ways in which you will inevitably have to deal with the fact that someone's perspective is irreconcilable with yours, despite the fact that neither of you are wrong. It's a tool to cope with life, not to run away with it.
With socionics, you can recognize when you need to stop preaching and let it be. And with socionics, you can recognize that some people can't stop preaching and let it be, that they have to find their own way to make peace with others' differences. Socionics can help you find the allies who are going to share your vision, and also to tell the difference between an enemy and somebody who just doesn't, and can't, understand.
Socionics is a great teacher of "not good, not bad, just different." Socionics helps us see the "just different," and even, sometimes, if you think hard enough, make some guesses as to what that "different" is like. And if you understand someone else's world, even a little bit, you can begin to slowly cross that gulf that separates your respective worldviews. And even if you can never touch (it's hard enough to touch with people whose worldview is perfectly complementary to yours), you might be able to get close enough that when you shout over the distance, the other person doesn't hear an unintelligible noise---they begin to hear language that they can translate back into their own, and begin to understand in their terms something like what you meant in yours. And then they can begin to communicate back to you in a sort of pidgin language of information. And eventually, it becomes easier to get productive work done with people with whom you have fundamental differences.
And yet you also know where to come back and unwind, where to find like-minded individuals. You can learn what topics and ideas might be too hot to touch. You can learn what approaches will cause more harm than good. You can learn to accept difference, rather than trying to prove that you are right. And you can also accept that some people will never accept you, because your view is foreign, contrary to all their modes of understanding the world, like if you were working as an architect and all of a sudden someone told you to sight read some music. You'd say, "I know how to read a blueprint, but this doesn't make any sense to me." And the musician would say, "But this is a blueprint!" You could reply, "No, it's not," and get into a shouting match. Or you could reply, "well, that's not the kind of blueprint that I'm used to, but let me show it to some of my friends, and maybe they can show me how this relates to the kind of blueprint I do understand." The latter, while more wordy, is more productive.
So, that's what the point of socionics is, in my opinion. And that's how we can begin to use socionics not just as a tool of intellectual masturbation (because lets be honest, this stuff is the softcore foot fetish flick of intellectual masturbation porn---works really well for some guys, but in general, not really gonna get you off), but as a tool for living in the real world, getting along with people, and understanding the world around us.