I'm not so good on rogues but I'll try to help with the priest.
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I was also thinking about the Primal Mooncloth set and was wondering if it was worth leveling tailoring for it
It's pretty much an essential if you want to get anywhere, progression wise, easily. The new 2.4 chest is impressive for the non-tailors but I still prefer the PMC Robe and what with the set bonus and combined stats, plus not having to put up with second best gear in the shoulder and waist slots, I'd say PMC is definitely worth it. In addition you get the Whitemend Pants and Hood, which while you don't need to be a tailor to wear you'll be losing the set bonus without it. Now 2.4 has arrived you have more choices gear wise but I'd still say tailoring for a priest who wants to make the best of himself pre-T5 is essential.
As for your second this would depend on whether you want to make money or have another useful profession for your priest and/or what your rogue will be doing. Depends on what you expect your priest to be doing, if he's going to be a raid healer then Alchemy is a nice one you not only get to make pots but you get a useful trinket... yes you could probably make more money from a gathering profession/skinning and buy the pots and make a profit over alchemy, but to be honest the Redeemer's Alchemist Stone (2.4) has such an excellent amount of +healing coupled with an on spellcast proc and better returns from mana and health pots that it'd be a tough call.
The main problem with taking Alchemy as I see it is it means either you make your rogue take Herbalism or spending a fortune on herbs to level it and ultimately you'd end up taking it for one trinket. I went Alchemy/Tailoring on my priest - dropping Herbalism once I'd levelled Alchemy, I considered dropping Alchemy altogether a few months ago but I'm glad I stuck with it now. :)