A Dedicated Crossbowman is a massively situational class. You qualify if you have over 150 in crossbow.
It's performance is based on:
1: The Weather. Due to rain, the class is crippled one out of every two games.
2: The Map and Gametype. A Crossbowman's ideal situation is a map and gametype that allows him to avoid melee, cavalry and have cover from archery the majority of the round. This avoidance allows him to actually be a crossbowman, getting off a reasonable number of shots before getting ganked.
The damage is justified by the immense loading time and crippling equipment limitations. You also exaggerate, the Arbalest does not one shot everyone and everything everytime. It's damage is variable to a great degree, as per example, I was unable to one hit kill a naked archer in the chest at 20m on three separate attempts. Yet a headshot under 10m on an Armet will kill two out of three times.
Rain is a retarded mechanic, in fact, it's the only thing balancing out xbows atm.
The map, well, every class suffers from that. You are clearly overexagerating the fact that rain would make you situational.
The damage is not justified by the loading time, and it isn't even to be called immense anymore. When talking about balance you have to take into account a character with at least a MW weapon if necessary, and I can tell you, Dave has oneshot me with his MW Arbalest a lot.
Don't get me wrong, I play an xbowman myself on my Magikarp alt, but I am appalled how effetive he is without even needing a high lvl and without any heirlooming.