That general statement is usually, but not always, true. In a few cases, a certain used stamp may be more expensive than its mint variety, but those instances are quite uncommon. Many great collections consist of used stamps only. Many collecting purists prefer the used specimens, based on the premise that stamps are created to move mail. To those collectors, a stamp without a cancel has never fulfilled its reason for existence.
Many great collections consist of used stamps only.
Used stamps are more plentiful than mint copies. You will have more to choose from, and they are easier to find. Used stamps, especially the older varieties, are far cheaper than mint copies. That's typical of the price relationship used versus mint on most older U.S. stamps. For the new or young collector just trying his wings in this hobby, most used stamps will represent no great loss if his collecting interests change, or if his lack of experience causes him to damage some stamps.