This would allow dispensers to place down seeds, saplings, mushrooms, chorus flowers, etc... Anything that can be planted and then grown.

This would aid in automatic farms, especially wood farms, without being so OP like dispensersPlaceBlocks. And compared to dispensersPlaceBlocks, it would probably interfere less with vanilla contraptions that rely on normal dispenser behavior.

In what way is dispensersPlaceBlocks more OP than the other blocks? If you do not want a block to be placed, you can just use a dropper, and arguably (at least according to some SciCraft players, and I agree), planting things is more OP than placing blocks, hence why there have been blacklists in some mods.

Secondly, why would the rule interfere with vanilla contraptions? If the item is to be spit out, then just use a dropper. If you are using a functionality e.g. shooting out projectiles, remember that these functionalities are either already for placing blocks (e.g. pumpkins on snow and iron golems, which won't be affected), and the rest are for blocks that cannot be placed (e.g. shooting out projectiles, placing water/lava, placing boats/minecarts on water/rails, etc.)