Short answer – Longevity. After talking with the company I bought the tile from, they suggested traditional mortar. They had actually never heard of mussel bound but said over time, a material like that will start to weaken as it ages AND if the wall does have even the slightest of moisture, it could release quickly. Mortar is also MUCH cheaper than mussel bound and even though it takes a bit longer and is a bit messier, I think its still the better way to go. Mussel bound is also in short supply in my city so trying to get some for the projects, could have taken awhile. Also, never use Mussel bound in showers. Showers should be sealed correctly and using the tile matt with compromise that protectant layer that should be beneath the tile. If there are any mini the grout, it could eventually leak through and weaken the mussel bound.