In a skid
airflow is from the outside, as you say. A crab is a cruise skid and has penalties in fuel and time, as you can imagine. In such a condition, there is no need (nor desire) for aileron. A slip is a descent device (forward) and very safe, though controls are crossed. I have slipped a small A/C with foot on the stop and stick full side. nose down, full flaps VSI 1200. A full slip at high speed and back stick is a lomcevak.
A slip is also useful in landing crosswind, though descent is to be monitored closely and Vs as well. One wheel landings are a thing of beauty, as is a well executed and abandoned crab at T/D.
rgds Airfoil