The thing is as with many adventure stories the plot is often what defines what can and cannot be. It's little different then telling superhero stories and depicting his abilities in an inconsisten manner.
If the Enterprise can still get to Delta Vega in a few days then it should have enough power to break orbit if need be. The same thing happens in "Mudd's Women" the the ship is dangerously low on power because of the burnt out dilithium crystals. What, no spares?
It's a plot device to create tension. A writer crafting a novel might think of his way out of this by finding a way to create peril in a way that's consistent with the ship's abilities. But on television and in many films they create a sense of peril where none should exist if they stayed consistent with what has already been established.
To me this speaks of the writers not yet really knowing the parameters of what the ship can and cannot do and/or they thought none of the viewers would notice such an inconsistency.