You have lots of flexibility to do as many or as few Must Haves and Unacceptables (and in any order) as you desire.
Unacceptables are not as aggressive as Must Haves for potentially eliminating many levels at once as not a consideration for the respondent.
For example, consider an attribute with six colors. If the respondent says Purple is unacceptable, then this just eliminates one color from consideration. But, if the respondent says they must have Blue, then that eliminates 5 colors all from that one mouse click.
We worry about respondents making decisions too early that would immediately knock out many levels at once as not possible for them. That's why we chose to do things the way we did. However, we are going more with what feels right rather than hard empirical evidence on this matter.