Numerous types of grasses and flowers can survive in deserts, so it would be a neat touch to be able to plant those in sand. As for the others, it'd be useful for building and decorative purposes to have the ability to place these in sand. It doesn't seem unrealistic for the player to be capable of planting them in sand either.
This would also help to partially solve a minor issue with world generation, as patches of sand and gravel generated underneath water bodies such as rivers often spread onto dry land creating beaches and river banks. Grass and flowers often spawn on these, but then break immediately when updated, which can be irritating. Having them be able to exist on top of sand would fix this for sand, causing it to only occur for gravel; allowing these four blocks to also be planted on gravel (which also doesn't seem too far-fetched) would completely fix this bug.