Personally, If I want to get my kid a smart phone it would be only for one reason, that is for him to get acquainted with the basic operations of the device, hence improving his technological knowledge. But anything apart this, I don't see the reason I should give a smart phone to my kid.
Considering how the web space is saturated with so much negativity, I would be the last parent to give a smart phone to my child. It is also appalling how children of now nowadays are more interested in things that would not add value to their lives than things that would better there futures.
The distractions from social media alone is not healthy for their cognitive development. The effects of all these things make it quite difficult for them to remain focus, which consequently would affect their overall performance in their academics.