I do not know enough about the enneagram system to know how 'arbitrary' it might really be, but in my mind any of the personality systems (including mbti) could be viewed as equally arbitrary.
For me personally, enneagram speaks to me in many ways more than mbti does. I'm very much one to go to underlying motivations in human behavior and personality - what drives each of us to do things, and why. I also think the disintegration/integration concepts are valid, and they speak more to the reality of personality - just that we aren't in one state of emotional/mental health all of the time - we might fluctuate. Personality isn't [usually] stagnant. Mbti doesn't account for that as easily. At least on the surface. I had 'a ha' moments months back when I was reading enneagram type 4 (which is probably my best fit) - but mbti descriptions often have been sources of confusion for me, which is why for a while I was really unsure what my type actually was.
And really..with mbti...the majority of people DON'T know the theory itself and have no idea about all of the cognitive functions, 'shadow' theory, and all of that. And....if your average person just takes the mbti test and doesn't dig very deeply, he's probably gonna be misinformed. It's only us weird ones who are really into it and are on a web forum about it! And we're definitely the minority!! So is mbti then even practical if very few people REALLY know much about it?? (and I'm not saying I know much - there are people on this site who know far more than I do about mbti!)