I've received this specific advice from legendary ultra-distance inline skater Eddie Matzger, who said that this very training has saved his hide many, many times.
From my own experience skating and teaching skating (inline, not board), one of the easiest ways to get injured in a fall is the "stick your hand out" reflex. Fall training mitigates this, ingraining habits to allow much larger areas of the body to take the impact, and ideally take it rolling. If you're going forward (doing the "superman") and have wrist guards or palm sliders, arms out can be ok. If you end up going more down than out (esp. many backward/side falls), it's likely a broken wrist or arm, right through any guard you might be wearing. (ask me how I know that one...)
From my own experience skating and teaching skating (inline, not board), one of the easiest ways to get injured in a fall is the "stick your hand out" reflex. Fall training mitigates this, ingraining habits to allow much larger areas of the body to take the impact, and ideally take it rolling. If you're going forward (doing the "superman") and have wrist guards or palm sliders, arms out can be ok. If you end up going more down than out (esp. many backward/side falls), it's likely a broken wrist or arm, right through any guard you might be wearing. (ask me how I know that one...)