The reason why you want to use transpose instead of just switching from Fire 1 to Bliz 1 is that you will drop your stack and loose out on bonus damage/MP regen until the next stack begins.

It's more efficient to use Fire1 until you can't cast anymore, transpose, and start Bliz 1 while building up MP faster since you'll be in Umbral before even casting Bliz 1. Once your MP is maxed you transpose from Umbral to Astral so your first Fire1 has bonus damage. As a THM/BLM you want to maximize the time spent in Astral and minimize your time in Umbral

Once you get Fire and Bliz 3, they will give you maxed out Astral/Umbral stacks when cast, letting you finally drop your transpose.