I would say that shadow is about 1.6 times better than a good holy/disc build for farming.
As to why people farm with holy/disc build, it's because they have no choice. If you only have 1 high-level character you pretty much have to farm with it. Respeccing to shadow every time you need to farm could get quite expensive.
You can ease the pain though, sneak searing light into your build, get plenty of water off a friendly mage, look at wizard oil and blackened basilisk, get a fair set of +dmg kit. get as good a wand as possible. Farm mobs that are signifigantly below your level. When I farm on my holy priest I pick on the Bog lords in south west Zangarmarsh. They die quickly, raraely chain pull and drop sellable greys, CE and sporregar rep drops and most importantly motes of life. I then sell these on the AH and buy what I want.