why would a customer not accept A105 valve material in lieu of A216 WCB. I thought they were pretty close to equal as far as material properties and characteristics.
For larger sizes a forging may be more expensive than a casting. A forging is a superior qualifying material. Will not entail additional heat treatments and examinations as required by castings.
What are those applications where a forging may not be used?
SVI answered this one pretty well. The only thing i would add is that, in my experience, A105 is generally used for small bore valves. A216 is for larger bore valves (> 2"). Service fluid, conditions and customer preferences should all be taken into account, here.