I have, on a few occasions, flown with a lap baby, but only when they are really small and pretty much sleep the whole time, like under 6mo. No way would I do a long distance flight with a squirming, heavy toddler on my lap. And even though flying is generally safe, turbulence or a crash landing can have a catastrophic outcome for children not properly secured. I don't fly that often, but I have had some gnarly experiences (sudden drops in smooth air, landing in a hurricane, etc) where no way I would want to be trying to hold a child. Now that we have several children, I doubt we will be flying very often because it gets very expensive, very quick, to buy multiple seats.
One other thing I haven't seen mentioned is that it's not really that safe to check a carseat, either at the baggage area or gate check, unless it is securely packaged in it's original packaging. If you watch how luggage gets thrown around on the tarmac, you can imagine what might happen to your carseat. Hairline fractures in plastic components compromise the integrity of the seat, rendering it less effective in a car accident. But I am a real stickler for carseat safety, extended rear facing, extended use of the five point harness, I only use new carseats, etc, some people don't care as much about these things.