Shortly after logging in for the first time today, I jumped into Arathi Basin for some honor grinding, and like a nub, I hadn't filled up on Soul Shards after my previous Shadowburning session. I had only 1 to work with, so there I stood, at the gate in AB thinking "Man, should I even summon a Soul Well for everyone?" But wait! I spotted the new Preparation buff that allows for "free" casting of spells before the start of the BG. /cast Soul Well. Everyone cheered, grabbed a Healthstone, and disappeared... just like my one and only Soul Shard.
Turns out that the Preparation time isn't "free" afterall, at least not for Warlocks. We are still being charged Soul Shards, which is odd since that is not the case during Arena game prep time. So, either Bliz is being really inconsistent or this is a little bug Bliz will hotfix shortly. My guess is the latter. But until Bliz actually does so, watch that Shard count of yours, unlike what I did today. =)