Some years ago an ex-oppo of mine told me about doing exactly the same thing as this in Mk4 Seakings - specifically hover-taxiing up the Pyg track in order to reach the summit of Snowdon in cloud as a training exercise. Apparently it was not thought particularly exceptional, but then it was the Navy.
I expressed surprise that even the Junglies would do this but he assured me that although hairy at first exposure (with an instructor) it was not all that hazardous as long as you remembered your escape routes at all times and flew slow enough to retain visal reference of the ground. I asked about hikers and he just grinned. Scared the s*** out of a few of those, he replied. I think he added that it prepared him for the conditions he encountered along the Basra Road shortly afterwards.
I wonder how uncommon this really is. Rather like SASless I'm sure many of us have moved aircraft on an airfield in bad vis. This is just an extension of that done off airfield by specifically trained military specialists for a specific type of task that most mil flyers don't go anywhere near. That's the point - specialists. Training for their specialisation. LIke mine clearance divers or HALO jumpers or smashing 20 tons of fast jet into a 600ft moving deck in fog iat 140Kts - or any of the other crazy things some get up to in the military.
Simply because there is a need for it.