In my opinion it it worth the money vs a carseat. I wish there was an equivalent that can be used in rental cars.

There actually is. There's the Ride Safer Travel Vest, which needs a tether or a shoulder strap. There is also a foldable high back booster seat. Forgot the name (anyone?) Neither can be used on an airplane but make good travel solutions.

By contrast, bringing a carseat into the plane is too much of a PITA, IMHO.

Bringing a car seat is actually not that big a deal but you have to be really organized. I traveled for many years alone with three children and a car seat on 11 1/2 hour flights with connections. If you have the luxury of flying with another adult, it's a no-brainer. There are though more "travel friendly" car seats out there that can make this a lot easier. For families who travel a lot, it's worth getting the right seat.