although you nailed the answer yourself there is exceptions to it.

for instance if you know an enemy or pack will die very shortly after but you run the risk of astral fire dissipating before you could transpose again to make sure you have full mp before next pull, you wouldn't want to use transpose in my opinion.

also certain section of boss fights have key moments that you could essentially limit yourself due to losing the ability to transpose leaving you to cast blizzard instead of just starting your rotation again.

that being said most the time transpose - firestarter is the way to go...