Very true. This all depends on the situation; time of day, where your other units are, where his are (especially if he has any more in the same locale as opposed to that one lone corps). The situation will help determine how one attacks and how one defends in the situation you describe. For example, and as charsep and I were alluding to in our previous posts, if an attack is made across an approach and the defender has one corps as the only units in the locale (and they're in the reserve),
and a second attack can be made across a different approach of that locale (even if only a feint), the attacker desperately wants the defender to defend with their whole corps, counterattack notwithstanding. They're committed to defending only that approach, once they declare it. If they do so, and then if even a lone cav unit from two locales away attacks (which would normally be a feint) across a second approach of the defender's locale, the entire defending corps has to skedaddle (and break apart, more importantly).
My apologies if you're already aware of this, as it does become very apparent very soon in learning the game.
But yes, it can be a long slog, two strong corps going at it across a locale.
There are of course many more subtleties to even just the defense declaration portion of the rules. If the defender breaks off only a couple of units to defend an approach (learning from a corps that had, in its entirety, defended a single approach only to be attacked through a second approach) and those units win the attack and stay in the defense locale (and no other units move in to support them later in the turn),
only those two units can defend across that approach; even if there are some huge corps in the reserve, they can't help defend, and if an attack breaks through those two defending units, well, see the shattered corps thing above.
This is a beauty of a game to behold.