It's not the reading before school that worries me but the teaching. My son was reading before school and a free reader before the end of rec but I did absolutely bugger all to make this happen. My other 2 are very similar. I simply went over the sounds correctly once or twice when asked and as a book lover have read masses of lovely books to them all.
The worry with the teaching is how it's done,great if you know what you're doing, are patient etc. Sadly not all parents know what they're doing,aren't patient etc. I had several kids over the years with well meaning parents who hadn't taught their kids correctly and had taken any joy out of reading their kids once had. They were very reluctant readers and several struggled.
I think pre-school readers just do it, they don't need to be taught. No 3 or 4 year old NEEDS to learn to read before school,great if they can but they don't need to. If they're early readers they do it naturally, if they're being taught at 3 they simply aren't ready. I never had to teach my son at all.
My son didn't gain anything from reading early. I read him masses of books that expanded his mind,give him pleasure and still do. He was getting that regardless of being able to read. Great he can read anything he wants to- woopy do-he is still the same boy in school absorbing the same things. All his class mates will be the same in the next few months so what's the big deal or hurry?
The most important thing is the fact he loves books. He said just yesterday"mummy I love books better than anything"that made me more proud than his reading ability. It will stand him in good stead for the rest of his life and will keep him on his journey as a free reader as the more you love books,the more you read,the more you keep your reading skills well oiled. I did that. Instead of killing any desire to read I gave him a love of books for life