Are cripple studs necessary for framing a door in a "decorative wall"
The cripples are typically a continuation of the 16" stud intervals. They make hanging drywall easier since an 8' or 4' length will fall in the middle of a stud on each end (with the exception of the cut end when your room isn't a perfect 4' increment). Like Shirlock says, they aren't required for non-load bearing walls (hardly anything is since the wall isn't load bearing), but as you remove parts like this, other details like the drywall and installing trim will become more difficult.