"In Wrath, the worse we had was 2.2,"
^ 3.2

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To give an example on how holding content back for PLANNED future content. Let's look at Wrath. It was, afterall, the most organised content schedule to date.

Imagine if, upon release, we DIDN'T have the Icecrown or Stormpeak zones. The landmass could be there, but you couldn't enter. They could have a zone-woide spell effect that prevents you from entering (IC=gargoyles chase you out/kill you with 5 seconds of entering. SP= heavy storm preventing access beyond the K3 area/questline).

Now, had this been planned from the start, the developers could've redistributed resources to giving us Azjol-Nerub as a zone as originally intended, but since the lack of two zones (and two dungeons) would've freed up more time than that zone would take up, they could've worked on Stormpeaks anyway.

Now, from there, imagine that 3.1 then had Stormpeaks and the Halls as additional content. Suddenly that patch is for more than just the raiders. It also gives us more to do whilst we wait for 3.2 which, due to IC being closed off and the addition of AN as a zone, could've been more interesting. A new area of the underground empire could open up, a Raid zone down there where you choose to either fight the Faceless ones or the Scourge. Then finally, 3.3, which would include the zone in addition to the raid and dungeons. Suddenly the year spent on 3.3 becomes much less painful.