IF it's tested now, there's not competition from players trying to maintain/make progress on the live server while also participating in testing and polishing on the PTR server...it makes the testing go slower.
IF it's tested at the end of Vanilla's phases, it will contain all the fixes that occur between launch and then...fixes which might cause more bugs for TBC content.
If I were in the meetings where the discussions on when to test TBC and Wrath content were taking place...those would be the main points of the discussion. Which would result in a more stable and polished experience for the release of the TBC content? Would the slower and less thorough testing at the end of Vanilla's phase fix enough bugs to meet the quality standards they want? Or will the fixes during Vanilla's live lifespan create enough new bugs for TBC's content that a TBC PTR now is pointless anyways?