There's no real benefit to soldering speaker cables, IMO. If I move things or want to change the length of a cable, I want to be able to remove the connectors easily. The connection point is still there, regardless of solder, so signal wise, it doesn't add anything. Strength and guaranteed connection are a benefit (providing the soldering is done well), but not enough to totally warrant the time to do it. That looks like a pretty decent solder job though.