Trees

Depends entirely on how you're consuming your herb. As mentioned to another poster, there are different grades (hardness) of steel. Cheap grinders don't hold a sharp edge on the teeth for very long. Your grinder still "works" but is it as easy to turn? Does it gunk up with resin fast? That's due to tearing/mushing the herb as opposed to grinding.

Additionally not everyone simply smokes their herb. Dry herbal vaporizers are essentially mini ovens designed to cook the herb to release the active compounds (without combusting and inhaling a nasty toxic cocktail of carcinogenics and carbon monoxide) - the fun part is different temps release different effects from the herb (so I can skip the sedative/couch-locking affects of the herb when desired). Not all dry vaporizers "cook" the same way - some devices work best with an ultra-fine grind (almost powdered kief) for optimal heat-transfer, while other vaporizers benefit from a very fluffy "airy" grind.

So if you're loading up a waterpipe or joint, sure a medium grind is good enough. But if you're trying to get fancy, there are certainly plenty of ways to enhance your sesh experience.