Even with a ship capable of overcoming the gravity wells present in the movie, it would likely still make sense to launch from Earth with boosters/staging to increase the amount of fuel/energy available to the ship for the rest of the journey.
Yes, that's a valid argument I think. On the other hand, I'd imagine if our technological level is such that we have SSTO ships (with SSTO even for more massive gravity wells than Earth's), we'd probably not use rockets but have another SSTO tanker fueling up the ship in orbit. You could maybe argue that ressource constraints in their dire situation have forged a situation where they put everything they had into those ships and the rockets were just what they had still lying around from an older age, so they made use of it. It certainly was another artistic choice in order to give it a more familiar feel and I can respect that.
A rocket-scientist (ok, aerospace engineer by training) friend was recently surprised that he hadn't realized this:
"If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."[1]
