How to Attach Spade 10 AWG Solid Copper

"and would REALLY like to know what could POSSIBLY be gained by crimping ANYTHING after soldering it"

1) Mechanical stability of the chemically bonded solder joint. Especially important in a speaker spade when you may torque down and remove an unknown amount of time over the life of the cable. Solder joints can and do fail under repeated load.

2) Metal to metal contact of the wire and connector, making for a direct contact connection. That is why I advocate to use a minimal amount of solder. You want to crimp onto metal, not solder.

That's dealing with butt connectors. I suppose you use those in your DIY audio cables too? =8^)

If you're referring to crimping an area of a connector that acts as a strain relief, it has no bearing on the actual connection/conductivity. Different story, and understood!