It hints at maintenance, a tune-up, a restoring of parts and replacing old ones. This in turn puts us in mind of care . When he urges us to keep language clean, it merely underscores the custodial relationship he's already described. Cleaning is maintenance. Finally, we don't have a mindless cleanliness, a Germanic-type of cleanliness for its own sake, but one for the sake of accuracy.
Precision is the ultimate goal.
Efficient, accurate, clean. These words carry with them a citrus-y simplicity, one that reinforces in my mind the virtues of harmony and balance. Impurities are removed. The most essential aspects of the language remain. Since we know that nothing is created or destroyed in a chemical reaction, we're left to assume that the impurities are converted by some means and made to stand in service to the word purified.