While that is true about the higher weight limits, I think that for most kids the higher limit is moot, because the benefit of using an infant seat is really parent convenience for moving them out of the car still sleeping and for taking them to places like grocery stores and restaurants before they can sit up unaided. You *can* skip the infant seat altogether and use a rear-facing convertible toddler seat from birth, and that is a cost-saving if you are so-inclined. With DS, I used my convertible toddler carseat from 7 mos until he was nearly 5, so I got about 4 years use out of it, both rear- and front-facing.
The average infant carseat weighs about 10 lbs. empty. I took my children out of them once they reached the height limits (5 months with DS, just about 8 months with DD -- we tend to be a long-waisted family); at that point they weighed about 14 lbs., and I was happy to stop carrying around the combined weight of the carseat AND the baby. I know that for myself, I wouldn't be inclined to want to carry around a 42 lb. load on one arm, so keeping the infant seat in use past the point where they can sit up isn't something I really care about.
Note that if your child outgrows the infant carseat for safe use in the car before he is able to sit up alone, you can still use it for convenience in restaurants and such until he masters that skill -- just keep it in the trunk of the car when you're not using it.