Human relations have the additional and curious property of changing the relationship through the act of labeling, and anyone who has ever said “I love you” can attest.
You see, you can't just sit back and say "let's not define X" just because it may change, or even if there is a guarantee that it will change. It should still be defined.
Social software presents users with a way to state that they have some relationship with someone. Some of the softwares even allow you to dinfe it in slightly more detail. We're simply trying to stabdardize and flesh out those relationships. There will obviously be some point at which new relationship types aren't added -- you have to draw a line somewhere.
As for ambivalentOf, my friend (and collaborator) Morbus suggested it to define his relationship with some guy who will go unnamed but is heavy handed in the RSS world. I put it in there because hey, you've gotta have fun at what you do. And that was fun!
Interestingly enough this schema started a year and a half ago (or was it more? I forget) as a way for me to begin writing a simple (ha!) genealogy schema to replace FamilyML, a simple genealogy schema. And now look where it is!
Ian has written more about Clay's writeup, with a bit more directed attention to the post itself.