Thers no getting around it, alchemy guru is required if you want to make a sensible approach to purifying lightstones, the guru level is too high, i know i would like to see mastery used, (like in hunting and some alchemy quest) or a drop in the requirement to purify.

I would say that keeping the lightstone conversion at Guru is important and neccessary, as there is no other reason to get to Guru, heck there is no other reason to go past Artisan Alchemist.  Guru Alchemy doesn't provide enough profit from imperials, Guru alchemy doesn't increase the chances of success to upgrade Alchemy Stones, Guru alchemy doesn't make it possible to mass produce, Guru alchemy doesn't create a 2nd perfume since perfumes are always a 1-1 creation and none of them can fail after you reach Artisan level.