Yes I agree with you in principle, and in fact in ARN we are not allowed oxygen charging rigs on the ramp. But the B744 pax oxy system has around 20 bottles. They are used in parallel, so that when the pressure is low you have to refill all the bottles at once. Maybe you keep 20 charged bottles in stock, I have one (for the crew). So if I had low pax oxy pressure on a B744, I would have to remove all the bottles and take them to the workshop and charge them one at a time. Oxy bottles charge very slowly because they can get hot. I would be much happier charging all 20 at once through an outside rig.
Could someone explain why Boeing would design such a heavy, complacated, maintenance intentsive and possibility dangerous system, when oxygen generators were available?