I think that this is tricky in K-12 Math in the current educational system. When factoring in the need to assess many students, mathematical procedural fluency is most easily assessed through written tests, which are easily copied or solved with an answer key. Adding additional tasks such as long written explanations disadvantages those students who may have strong mathematical skill but poor language skill. Although, mathematical communication is also important, it can be done purely symbolically, and this method of communication is usually most like how we use math outside of school. At the same time, we must assess procedural fluency. Without more time spent on Math and more Math integrated into other subjects, my experience is that students don't acquire adequate fluency without motivation to specifically practice those skills for assessment.
Then again, perhaps tests which are protected against copying and cheating, such as through invigilation, are not the tasks referred to here...