Good call, spriest silence is not an interrupt.
And I was confusing purge with earth shock which isn't in vanilla, its a TBC thing.
And as for the meditation thing, your spirit tick will always continue, even while casting, it never becomes interrupted ~ just changes values at different points in time depending on how much spirit you have, 1/3 or 3/3 meditation, and/or Tier 2 three piece set bonus.
Edit to add:
It really becomes fun with spirit tap and the blue dragon card trinket.
Spirit and Mp5 are two separate forms of mana regeneration:
Spirit = xMp/Tick (Two Seconds, like Rogue's Energy) and 1x Mp/ Tick = 4 Spirit
Mp5 = 5 Seconds.
What makes them each unique in their own perspectives is that Spirit is effected by Kings, makes +4 Stats on Chest worth it, and scales as a % so the more you have the more you get. However, Spirit is effected by "Combat Casting" where you regenerate mana after 5 seconds from casting a spell, there are a few ways to "modify" this rule with talents and gear.
Mp5 is not effected by any spells, abilities or skills in the game. It can not be buffed or cursed negatively, it is not effected by the casting rule either, which is what makes it the best mana regeneration in the game for all classes except Priests.
Using 5/5 Spiritual Guidance (your 21~25th Holy Talents) you are able to transform 25% of your spirit into Damage & Healing. This means that now what was once 4xSpirit=1xMp/Tick is now also 4xSpirit = 1xMp/Tick + 1xDmg&Healing. What is most unique about this is it is a flat bonus to Dmg&Healing and not "Up To"; so your spells will have a permanent increase in base rather than a dice roll.
The maximum possible explanation of this is take the Priest Tier 3 setup, any rings, neck, cloak or weapon of choice, and your "Dark Moon Blue Dragon" card procs, which is 2x Spirit in combat along with normal mana regeneration, this also stacks with spirit tap, and would easily give a Smite Priest over 1000 Spirit, or 250 Dmg&Healing + 250 Mp/Tick.
Don't forget Innervate ;)
Heal Rank 2, Heal Rank 4, and Greater Heal Rank 1 "Spam" @ 2.5 Second Casting Speed which = The Average Attack Speed of most Bosses in the game. Also granting the +25% Armor Buff from Inspiration.
Full Tier 3 Talent Spec w/ Smiting: