I don't know any LEOs that still carry the 20xp "tank". Too large, try sitting down in the car with it in your pocket or on your belt. Ouch. Stinger or SFs around here. I hardly ever see the magcharger anymore (on LEO belts).
I like the external battery charger so I can do a full drain, charge cycle rather than putting the light on the charger half charged and risk taking it off before full charge.
Kind of like a cellphone, I hate it being tied to a wall while charging. A battery brewing in the glovebox, and a quick 10 second swap works much better for me. I guess that's why there are options, people's taste differ. I hope they offer the piggyback charger with the new SL. Best of both worlds, drop in and external!