I don't see them scaring massive spiders... though it would be nice if another mob in general had that perk... but I think them being able to travel far distances to deliver either a single item or book would be neat feature.
Perhaps if Bundles were to be reenabled (by this or another mod), it would be a perfect way of sending more than just one item.
In very late game you could send Shulker Boxes using Pigeons too!
They could be sent to specific blocks which can store items and could be interacted with Redstone.