It doesn't make sense to me to lose the rich semantics of UAVO fields. I prefer to adjust the computer to the human instead of the human to the computer. Another thing to think about is if we think we will have checkboxes even if the UAVO takes more values. I can think of at least one example where the user would only ever use two values, even if we track 3.

I agree about it being complicated and confusing. A lot of that could be improved with proper comments, but that's outside the scope of this discussion.