Thanks to Chaos Orbs in Cataclysm, which began life as a BoP item, but became BoE with the last patch, and Wrath of the Lich King's Frozen Orbs, many had assumed that Spirits of Harmony would lose their soulbound status towards the end of the expansion. Community Manager Lore has confirmed once and for all that there are no plans to make this change with Spirits of Harmony.
We're not entirely sure on the reasoning behind this decision, but it might be something of an annoyance for players. Although, with previous expansions at least, the item was largely completely irrelevant by this point. Spirits of Harmony, in so much as they can be traded for items in the Shrine, have at least held some value. So don't stockpile them for an alt's crafting, on the basis that they might become BoA, or for an auction house bonanza that may never come.