The main reason they solder the connections is reliability. In production, this doesn't take much time but for the weekend warrior or someone who just hasn't come close to mastering the task of soldering, it's the worst of all worlds. In production, it's a lot faster than attaching terminals and connecting them. For the end user, it would be great, but the cost would be reflected at time of sale.
The Markertek parts are for solid wire, not thin stranded wire. What would be a more workable solution is called a Phoenix connector and it squeezes the wire between a thin metal strip and a hole in the metal, making it harder to damage the wire. These are often used on commercial audio equipment and are the replacement for screws and spade terminals and they're in the Duncan video.
Size may be an issue, though. A soldered wire takes up no extra space.
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