Evan,
Flying a pattern with the intention of 50% spoilers is to provide the safety buffer in both directions (high and low), rather than needing to adjust your ground track and preserve the sacred rectangular pattern. While you are completely correct that most spoilers can cure your excessive altitude problems in the pattern, if you fly a pattern that is sized to not need any spoilers/airbrakes and you are too low, you must now change your pattern rather than closing the airbrakes a bit. It is mostly about making the size of your pattern halfway between what would be required for a no-spoilers pattern and a full spoilers pattern.

Chris