There are many aspects of practical exams that cannot be fulfilled using the implementation of discriptive method in the same.

The practical points cannot be expressed in a way they actually meant.

We're already given discriptive exam earlier, if we have to give same in practical whats going to be different in the answers of theory and practical then.

If it's a need to take practical exam, it's better to choose viva as an option because that's more appropriate as students will be able to explain the practical methods more precisely and clearly.