Points in favor of giving Reimu a voice:
El Numero Uno: The Magic of Interpretation
As mentioned before, part of the magic of the Touhou Project is the interpretation of not just the characters, but in the story and world they're in: you'd be surprised to find just how many interpretations are in even the slightest details in Touhou. From the over-the-top comedy of Warugaki to the gothic setting and dark interpretations of Koumajou Densetsu, the interpretations are through the roof. If Sakurai were to put Reimu in Smash, then it would be HIS interpretation of the world of Gensokyo and its inhabitants, and his alone.
El Numero Dos: Other Mute Characters in Smash Have Voices
Ever since Smash 64, normally-mute characters have been given talking voices. Ness and Lucas. Captain Falcon. Pit. Meta Knight. Marth and Roy. They normally could never talk by the time they debuted in the Smash series, yet Smash seems to have greatly developed their vocal cords. Hell, this is still a thing that happens today, with Arusu, Solo, and Eight, alternate costumes for the Hero, getting their own voices in the mix, too! Reimu having a talking voice would not be too out there as far as I'm concerned.
El Numero Tres: Reimu Has Had A Voice Before
Believe it or not, despite not being the case in the official video games, Reimu has been given VAs in other Touhou-related media. Mai Nakahara in Summer Day's Dream, Kanzaki Kanari in Touhou M1 Grand Prix, Asami Imai in The Sealed Esoteric History, there are a good number of VAs who have provided their voices to Reimu, because since it was their interpretation, they were free to give Reimu a voice. Sakurai giving Reimu a voice would just be par for the course in terms of the many interpretations of Touhou.