If he wait final.QC it means that you have your normal first round to cast spawnpoint/vinetree.
Let me back up a bit and explain myself a bit more fully.
There are several variations on a Priestess casting DI on the enemy mage turn 1. If she has initiative or not, if she waits until the final QC phase or not, etc. The point I am trying to make is that responding appropriately to each one requires preparing a different set of spells in rounds 1 and 2, and there is no way to determine with certainty which set to prepare ahead of time. The opponent has to guess. If he guesses correctly, great. If he doesn't though, or if initiative is such that it doesn't matter, the game may as well be over at that point.
So when I bring up a specific situation as an example, and you give me the correct counter-play to that example, you are missing the point. The point is that before the Priestess does the DI trick, before spells are prepared, before mages are revealed, before the players even sit down at the table or computer to play, the opponent of the holy mage is severely disadvantaged because of this ruling related to DI.
Some books aren't affected very much by it, but the ones that are can and will be shut down very quick. Not because of an overall book strategy or because of skilled play, but because of one card. One single card, the counter-play to which is guessing right during two Planning phases in a row and getting lucky on initiative.
And I have not even gotten started on this spell cancelling a Grizzly or Thorg in the midgame.