Mochi: pounded vs. from rice flour
A cooked rice grain is not an evenly cooked product... The center has a different consistency to the outside. Pounding the rice would combine the stickiness and would activate rice gluten that is present. 

I would assume using rice flour in the recipe would be like using a different grade of flour in a recipe... The texture would be different, more or less sticky, more or less stretchy...

Malaysian cakes are improved by adding different grades of rice flour as well as adding other types of flour to the mix. And freshly made rice noodles made from ground rice; as well as Vietnamese rice paper noodles taste better when the base if 100% steamed rice instead of a mixture of rice and rice flour.

Mochi which is as much about the flavor as well as the texture I feel would benefit from the process as much as the quality or processing. 

As an aside, why does hand folded and spread pizza dough taste and feel so much better than a machine made pizza base? Same ingredients, different process no?

Making Rice Paper In Vietnam: https://www.youtube.com/watch?v=HaTr-DPC5yo

Making Mochi in Japan (where would it be without the theatre?) https://www.youtube.com/watch?v=olTuSpJTL2g