Our take is this and has always been this - it's about combining profitability with logic.
Point a. Shipping direct is not scalable, what if you had a product or a number of products that start to sell really well then the most profitable way to ship is by sea - therefore you cannot ship directly to Amazon by sea.
Point b. Shipping by air direct does come with risks - for sure the labels being removed or labelled over is one - but also you are at the mercy of couriers not bashing your goods around (shipped to a hub you have a second chance), once they land into Amazon they are in - possibly in unfullfillables.
Point c. Shipping direct in the European Union is a Tax nightmare waiting to happen - been there done that - had goods "stranded" in Germany - never again , all to save single figure points on the dollar.
Point d. Who is the recipient of the goods,and who is declaring the goods (if shipped via a Chinese supplier an agent will be allocated to the shipment upon arrival - the price of this agent will not be included in the Chinese suppliers "shipping" quote). This cannot be the hub as the goods are not invoiced to them. Nor can it be Amazon as the goods are not invoiced to them (Importer of Record)
Point e. While the supplier may be offering you lower "SHIPPING" costs they may not be paying all the fees associated with shipping. Especially if the quantities are low , then an additional $0.47 per unit for piece of mind, ease of shipping and knowing that this is the "ACTUAL" price for shipping. This is why we use and recommend a freight forwarder every time.