It's all a matter of opinion of course. In my opinion from an acting perspective I would prefer a child who embodies the role emotionally verses a technically perfect singer. Yeah, but I gotta agree with you that it does seem odd to me that from the entire talent pool out there one couldn't pick an Annie that is both talented vocally and acting wise. (said from an objective standpoint. I have not heard the current cast)
But then again if we're going true to the show, these aren't perfect little Shirley Temple trained vocalists. They're supposed to be orphans. Raw and untrained with a little edge. Which makes vocal flaws endearing and true to their characters.