I am borrowing this from my friend Amy over at 
Family Journeys with Twins....This is why I am planning to keep Charlotte rear-facing for as long as we can! :) We worked too hard to get her here safely to lose her over something so silly as her legs might be a little squished. So, until she reaches 35lbs, the weight limit for our convertible seat, she will be rear-facing!
The American Academy of Pediatrics recommends rear-facing for as long as  possible for the best protection. This means that children should  rear-face to the maximum limits of a convertible carseat...which is  30-35 lbs. (depending on the seat) OR when his/her head is 1-inch from  the top of the carseat shell. The rule that most parents know is that  children must rear-face to at least 1 year AND 20 lbs. (the child must  be BOTH 1 year and 20 lbs., not either or). What is not well known is  that 1 year AND 20 lbs. is the bare minimum and it is strongly  recommended that they be kept rear-facing for much longer.
When a  child is forward-facing, there is a lot of stress put on his/her neck  in a crash. The weight of a child's head in a crash causes the spinal  column to stretch...the spinal cord, however, is NOT meant to stretch!  The spinal column can stretch up to 2 inches but the spinal cord can  only stretch up to 1/4 inch before it snaps, which means paralysis or  even death. This is referred to as "internal decapitation"...the child's  head would be slumped forward and it would look as though he/she was  sleeping. It doesn't matter if the child has great head control...that  means nothing. Data is showing that a forward-facing child is 4 times  more likely to be seriously injured or killed than a rear-facing child  of the same age.
Rear-facing seats do such a great job of  protecting children because the back of the carseat absorbs the crash  forces. The child's head, neck, and spine are kept in alignment,  allowing the carseat to absorb the forces. The child's head is also kept  contained in the carseat, decreasing the risk of coming into contact  with projectiles.
Something I hear often is "His legs are  scrunched up, he must be uncomfortable" or "Won't his legs be injured  that way?". There has NEVER been a single reported case of hip/leg/foot  injury from extended rear-facing. Even if there were...a broken leg is  much better than a broken neck.
Children are much more flexible  than adults, so what may be uncomfortable for us, is not for a child. If  you watch a child playing, you will notice that they choose to fold  their legs up...they don't sit with them straight out or hanging over  the edge of the couch. My boys fold their legs up in the stroller and  squat or sit with their legs underneath them when playing quite often.  They have never once complained about being uncomfortable in their  carseats...never...the just fold their legs, hang them over the sides,  or prop them up on the seatback. That is comfortable to a child.
Just  the other day, when we got into the car, my 3 year old got in his  brother's seat (rear-facing) and refused to get out. I asked him several  times if he was sure he wanted to ride that way and he said yes. After  being forward-facing for almost 3 months, he chose to sit rear-facing  again and didn't complain once. He's hovering around the rear-facing  weight limit though, so I did turn his seat back around forward-facing  today.
As far as being harnessed as long as possible, think race  car drivers. They wear 5 pt. harnesses, not just seatbelts. A 5 pt  harness is much safer in side-impacts and rollovers, which tend to be  very serious/deadly crashes. Most children outgrow their carseats (most  go to 40 lbs.) before they are truly ready for a booster. The bare  minimum for a booster is 4 years and 40 lbs...a child should be in a 5  pt. harness until then and even longer if possible.
Even at 4  years and 40 lbs, a child should only be moved to a booster if he or she  can sit in it properly for the entire trip, every trip. It is extremely  important for the belt to fit properly and stay in place (lap belt as  low as possible, touching thighs, and shoulder belt between neck and  shoulder).
The 5-Step Test.
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
If  the answer is no to ANY of those questions, the child needs a booster  seat. A 4-8 year old child in a booster seat is 59% less likely to be  injured in a crash than a child of the same age wearing a seatbelt  alone. On a side note, a booster must always be used with a lap/shoulder  belt and it is also important than any person riding in just a seatbelt  wear a lap/shoulder belt, not a lap belt only.
You can't control  how others drive...accidents happen, it's just a fact of life. So why  not keep our children as safe as we possibly can...that is one thing we  CAN control. If I am in an accident, I know that I've done everything I  can to make my babies as safe as they can be in the car. I know that any  injury they have will not be because I did not protect them and I can  live with that!"