Can't think of any profession they wouldn't be advantageous to over a standard crossbow. The simple fact that you can cock them and stay hidden makes them a must if you are sniping. Your example of casting 608 and cocking in the round time only works with a mechanical crossbow. A regular one would knock you out of hiding. Then with a mechanical crossbow you can cock and fire again from hiding which allows you to keep your AS boost from 608 for multiple shots on a single cast.

Now if you want to debate crossbow over bow then there are too many pros and cons to really make an absolute decision I think. Personally I'm tempted to roll up an elf or dwarf ranger and switch to bows just for the 3 second fire times. However, my halfling ranger is pretty damn deadly with a heavy mechanical crossbow.

For early levels 0-15 I still suggest sword and board or a claid.. Crossbows will be deadly, but unless you have a pocket wizard you will spending most of the time dead yourself.