Here is one of the reason's that I would be willing to spend more on a seat...
Comfort!
Yes. Comfort. When you or I go out to buy a chair, sofa, bed, etc. that we will be using a LOT, we sit in them and base a lot of our choices on how comfortable they are. If we are asking our children to sit in a seat for 2-5 YEARS for long periods of time, then heck yeah I want my son to be comfortable!
It will mean I'll fight him less getting him into the seat, he'll stay comfortable and quiet on car trips, and he'll be more likely to sleep for a larger portion of the trip. That makes my son happy, and his father REALLY happy and much more sane! I mean seriously... Let's take a look at one of the thinner padded seats. Say a Scenara. There is MAYBE a quarter of an inch of not very soft padding on top of a hard plastic shell. Would YOU want to be strapped into that seat for hours at a time, unable to move? I know my ass would fall asleep after only about 30 minutes.
Now let's think about the summer or winter in that seat... Plastic does a really good job of getting either really hot or really cold, and then holding that temperature for a long time. So all there is between me and a hot as hell, or cold as ice hunk of plastic is the aforementioned maybe quarter inch of padding? And I'm supposed to enjoy that? My son is supposed to be happy with that? We amortize out the cost of better seats over multiple years in order to justify higher costs for safety features, so why the hell don't we do the same for comfort?
Me personally? And I'm not trying to question anyone elses parenting decisions here, but I just can't see sacrificing my son's comfort and my sanity for a measly $30 a year... ($150 / 5-years using the Nautilus vs. Frontier scenario.)