Relationships take hard, hard work and require a high level of commitment. If both people are willing to work at it, I don't see why some of the worst-suited types should think twice about starting something.
BUT, MBTI certainly has its commonsense applications. An INFJ, like me, for example, can use it to help them find/narrow down what kind of job they'd be suited for as opposed to jumping into some horribly suited job, like accounting.
I'm still uber-new to MBTI but it seems like if someone spent a good amount of time observing themselves, how they act around others in different situations, likes and dislikes, that they'd be able to come up with a rough (and I mean ROUGH) understanding of how their personality works.
The nice thing about MBTI is that someone's already gone through the trouble of all the head-work. We get to take the test, and go 'Oh! I'm a ____.'
How seriously should MBTI be taken? Well, someone could conceivably, if they already haven't, come up with a similar test that tells you what political party you're in. But if I took such a test I would certainly take it with a grain of salt and make my own decisions given the situation at hand.
Similarly, I think MBTI should not be given a status of infallibility, but should be used as a self-discovery/learning tool that can be deviated from at any time.
No matter how you look at it, though, it's thought provoking and addictive as all get out.