You need to look at the consequences one at a time.

• If students use e-books they they don't forget their books.
• If the teacher distributes a pdf then the students don't throw it away.
• Many students cannot type fast enough to take notes on an ipad.
• You lose some students to Facebook, but at least the horizontal screen make it easy for the teacher to see what the student is doing.
• The students need access to a real computer if they need to work on a long report or essay in class.
• The students probably need access to a real computer if they need to work with spreadsheets.
• If you install a browser with flash then you can use interactive websites to add variation to the lessons.
   

I am a high school teacher so some of these points may not ne relevant to young children.