I just scanned through the thread and see now that we are back to the same age old debates
I will chime in with basically the same things that many have said. Perhaps my experience being a mother, who flew with carseat aged kids about 6-10 times per year will carry a little weight. Who knows though
Chidlren are at more risk while riding in the car to the ariport, even while confined in carseats than they ever arewhile flying in or out of a carseat. I figured if I am willing to take the risk to drive then I should be okay with the infintessimal risk to fly. The lap belts always held my childrn fine even through very rough turbulence (which we did hit a couple of times when they were tiny--and DS has always been a super skinny kid). Also, odds are HUGE that IF there is a real problem in the flight either everyeone will perish (carseat or not) or everyone will make it. Furthermore, the few serious emergencies which end up with living passengers tend to need a quick evacuation of a plane as it fills with smoke. It is much faster to get a child out of a standard airline safety belt than a car seat (and faster to get out of the row without a seat in the way).
If you really want to increase your child's chances of surving a crash, you would be much better off to sit near an exit row and in the rear of the plane (estimates are your chance of survival is up to 40% higher in the rear of an aircraft) than to worry about the carseat (personally I have decided airline travel is a low risk and do not worry about it at all). DBiL studies ariline crashes and design airbags for planes for a living. He is an expert on these matters and ends up quoted in meadia outlets after just about any crash or near miss. He agrees with my thoughts on this and did not use a carseat for his son on flights either. All of this to say--I truly feel that the "child safety" argument here is serious overkill (and I am very fond of safety, really I am).
Okay, so deciding that the carseat merely provides a perception of safety (not talking lap child here--talking about being restrained in a lapbelt vs. a carseat) I prefered lapbelts for several other reasons as well:
Frist, as stated here many times, it helps keep the child from kicking the seat in front of him. A couple of kicks or bumps here or there followed by a quick apology are to be expected and only the most sour of travellers would hold that against someone. However, repeated kicking and/or kicking that a parent does not try to stop is terrifically rude and really not acceptable behaviour. Period.
Secondly, I think many kids are used to a certain set of behaviours in the car. The carseat singles to the child that the same rules apply. Those rule may include getting to watch a movie to entire ride, the ride lasting less than an hour, talking in a "loudish" voice to be heard by parents in the front seat from the back, etc. I find having the different seat belt helps send the signal that this is a different enviornment and can help the child remember to keep a "library" voice, cope with not being able to use eletronics during take off and landing, etc.
Finally, I had my children's comfort in mind. Just as adults tend to prefer the roomier seats in first class, I think my chidlren prefered the extra "wiggle room" provded by having the full seat (and prefered not to be squished if the passenger in front of them reclined). Car seats hold a child really pretty tightly in one place and position. I think this is acceptable for shorter car rides (and needed as it does add a great deal to safety in such places) to restrain a child to such an extent, but not for the prolonged duration of a flight.