It is "rare" because it drops randomly from the "random rare weapon" build option in the Weapons Depot. No one is saying that it is "rare" to have, and in fact (as you so astutely noted) is is in fact a common drop, not only from the job itself, but also just from robbing and fighting. That said, even if this particular weapon is pretty common, it is not a fair assessment to say that any loot that drops from jobs is automatically "not rare." Take some of the Bangkok HEL, for example: Titanium Katars and Chain Vipers have a base drop rate of around maybe 2%-3%, so even after the Mastery bonuses and the Daily Chance Collection vaulting bonus (which is itself pretty damn hard to get, and in this case supplies approximately 2/5 of your overall drop chance!) are applied, your drop rate is around 27%-28%. So it would take in excess of 200,000 energy to accumulate 500 of these items, if the median job cost is over 100 energy. That's no simple feat! Thus, certain items, as determined by their low base drop rate and the possibly high energy job costs associated with them, is "rare" in comparison with most loot.