If you use pex for the tub spout when you turn on the water to the tub water will back up through the shower head due to the reduced inner diameter of the pex it s called stacking.
Why can t you use pex for tub spout drop.
Long story short it is unacceptable.
Copper would help give more support to the drop ear fitting and tub spout which is probably why they recommend it.
With a built in diverter you could use a pex drop eared ninety degree female adapter and a nipple but that is more trouble than it is worth.
Use a drop ear bend support to hold the tubing in a tight bend right.
Copper is the only professional installation material.
You can connect a compression type shutoff valve to the one half inch copper stub out and then connect your fixture.
The interior diameter of pex is actually a fraction smaller than copper this forces water to come out of the shower at the same time as the tub spout.
2 reasons i learned.
A lot of manufacturers are very specific about this and i ve seen it hundreds of times.
How did this project turn out.
In areas that are concealed like under a kitchen sink or vanity cabinet you can eliminate a joint by running pex supply directly to the shutoff valve.
Why can t you use pex for a tub spout.