Don't have too much knowledge on Cup-bearing (apart from my days of service
in St. Patricks), but I know that the cup, being a vessel for holding liquids
(namely water) is symbolic of that element, in ritual or ceremony. In Druidic t
erms (which I can speak for) the chalice, or bowl is placed in the circle at th
e 8 festivals, and if size allows, is borne by a 'cup-bearer', like the sword-b
earer, the rose-bearer, etc.
In any monarchy however, the cup-bearer, seen from the viewpoint of simple
servitude, is important in that he/she was often the poison-taster cmobined.