Is it OK to use a plastic trim piece that came primed without painting it?

Maybe the better question would be, without painting it will it eventually start fading since it'* primed plastic?

I’m happy with the primed color (charcoal) and could use it as it, but if it’* going to end up fading and looking bad later then I’ll need to get it painted.

This is a question of time. Primer won't last as long as final paint. Maybe try clear coating it.

Yep, I'm planning on having it painted at a paint and body shop... hopefully "Professionally"

Is this plastic exposed to direct sunlight?

Yes, it'* a plastic trim piece on my '09 Colorado pickup.

Even though it is garage kept and not driven daily, I'm going to go ahead and paint it as it would have been had it been a factory piece.

Just so I won't have to take it off later and paint it.

Since you are going to do anything with it at all you should do a paint and clear.