Pros? Sure, you have a child you can show off their skills, a child who responds quickly, who likes to be rewarded and please you. Your child can recognize words, objects, foreign languages, possibly even math problems, spew out several facts about a person, place or thing. They might enjoy it. You have a head start on 99% of the children the same age. You have a smart kid. The child can develop learning/studying techniques (but they won't be used for a few years). And they'll probably read chapter books before 2nd grade.
I have never pushed much of this on Kian and yet at three he recognizes some words, he knows all his pre-academic skills for his age and beyond (ie. colors, shapes, counting, etc.). The other day he surprised me by sounding out the word "eggs". He asked me what the word said and just for the heck of it I asked him "what sounds do the letters make?" He said "eh, guh, guh, suh" (try explaining why you only say one 'guh' even though there are 2 g's!) after a few times he figured it out, "eggs!". This is from everyday play and talk (and thanks to his grandmother for several Leap Frog letter toys, which I cringe everytime I hear that darn song "e says eh and ee"). Sure I inject educational quips into everything we do (counting items, pointing out signs at the stores on the foods, discussing details about objects and processes-road paving for example). This leads to a better understanding of their world they live in and expands their interests.
Kian knows what store we arrive at, and recognizes the bags from each store. It's not really "reading" per se, it's shape recognition almost. How do we tell someone to write a letter? By describing the shape (A, it's a slanted line, pointed top, etc.). The same way the kids learn square, circle, etc. they learn that letters have shapes. They learn that every time they see that B shape you say Ball and they see a picture of a ball, and so on. I've just seen clients who couldn't speak a real word, but could "read" and while that was great, it didn't help their communication. They still couldn't tell mom they wanted juice or crackers. They still couldn't say "help".
It's about balancing it with their cognitive developmental needs and abilities. For children that enjoy it, it's great, especially if all other developmental areas are age appropriate. However, it can turn into an experiment on the child. Most parents end up using rewards (toys, praise, food, etc.) to get the results they want. They praise and offer a treat when the child performs the task correctly. The child picks up on disappointment and they just really want to please you, and might not really be learning what you're "teaching" but rather figuring out what pleases you (which isn't all terrible, we use forms of operant conditioning to get rid of bad behaviors and encourage good ones).
The real concern I think is that these kids might be learning words and other things, but you're going to send them into kindergarten (unless you plan on homeschooling, this will happen) with 23 other kids who cannot recognize a word, except maybe their name. They won't know vehicles, birds, countries, etc. and your kid is going to be really, really bored. Kindergarten is about circle time, socializing, learning to stand in line, what it means to be in a classroom. Not much fun for little Maggie who already knows the theory of relativity.
Another thing is that you're not tapping into the child's full range of sense. You're only engaging one sense, there are so many other sense to involve in the process of learning. A child learns best when all 5 senses are engaged in the process. Touching, smelling, even tasting are as much a part of learning as listening and seeing. Think mud, playdough, cooking, sand, water, etc. Music and play, pretend play, encouraging imagination and a wide variety of interests along with social exposure is the best way to develop a child's skills all around.
If a child is interested in things they learn about them, with more vigor. Kian loves Cars movie and construction vehicles, and about 400 other things. Having a limited interest can be a concern. The extreme giftedness can sometimes cover up other concerns and disabilities. Many kids on the spectrum are very talented in certain areas, but have social issues. All kids have a favorite thing, cartoon, etc. And they know all about it, every detail and will tell you. But, when a child cannot deviate from that favorite, it can be a concern. All kids have their strengths and that makes it more fun and interesting in preschool when Johnny can tell you about bugs, Mary can tell you about flowers, and Joey can teach you about music. This makes life diverse and more exciting for sure. But really, all kids will learn these things, eventually. Let kids be kids. Why the rush?
"In our country there seems to be an unspoken competition among parents. We feel as if it is somehow a reflection on our parenting skills if a friend's child learns to roll over, walk, talk, use the potty, or read before our child does. Isn't that a bit ridiculous? Do we feel that we don't quite measure up if a friend can paint a beautiful picture while ours look like hen-scratches? Of course not. Different people have different skills. We, as parents, need to make sure that our kids have a chance to follow their own pattern of skill development." (found here)I know many of you disagree with what I say, we each parent differently and that's okay. But since I was asked a few times in the last few weeks, there's my opinion. Even as adults we have different skills and strengths. Life would be very boring if we were all just alike. It's just about being cautious and not pushing too much, too fast, and possibly causing children to dislike learning later on in life.