Right, but once you get increases in energy requirements, you also change the passage of time for the travelers. So the figures are a good way to show that at 1 g acceleration you can achieve noteworthy time effects after just one year of acceleration.
There is another factor too, although small but still a factor. Not to be annoying here but bringing up how engines work.
Lets imagine that fuel is not an issue here. You have plenty of fuel.
The way an engine works is it is essentially spraying out matter out the back and this causes the ship to be "pushed" in the opposite direction. Simple, you probably already know that. Obvious right?
Well the matter being sprayed out needs to actually travel faster than the resultant thrust. Did that sentence make any sense? The "exhaust" coming out of the engine moves faster than the resulting thrust experienced by the ship. Its not perfectly equal. You get some loss because the exhaust isn't limited by mass where as the ship is limited by mass. So the exhaust moves faster. Hopefully that makes sense.
Now, if a ship needed one last drop of thrust to reach C then it also means the exhaust coming out of the engine would have to move faster than C. But it can't. The exhaust can not move faster than C to allow the ship to obtain that last little bit of thrust.
So even if you could technically make matter move at the speed of light. We would be limited by the way engines work.