That is an interesting point. I only learned to read when I was 11 (never had any need to, before that) which is strangely late. When I went to school at 12, I found I was the fastest reader, and tested as "University +" reading level, which is the highest there was.
By learning independently of school, I learned way later in life, and learned in a way that was right for my stage of development.
My anecdote about it not being productive is purely subjective, but the science on development theory is clear. If left to their own devices, almost all people learn native language, writing and math on their own timeline in about 100 hours. School makes this process far less effcient, and teaches things like grammer that program the brain to read slowly and without depth.
Obviously my anecdote is hyperbolic, I speak primarily as someone who had a great unschooling experience and then went to school and saw the difference, and a little bit as someone who has read extensive child development theory.