If done correctly, twisting wires together can create a surprisingly solid connection but there's a lot of potential for sloppy work. Back in the day this was common practice in many (though not all) European countries, especially before WWII. In Hungary it stayed until after the wall came down, scarily enough with aluminium wires. Surprisingly enough the country still hasn't burned down, even though plenty of those old splices are still in daily use. In several former Soviet countries this method still seems to be fairly popular, although self-respecting electricians solder, weld or crimp to create a permanent connection.
Actually, there's an even worse method of connecting household-size mains wires though, historically speaking. Bend the ends of all the solid wires you want to join into eyelets using needle-nose pliers, push an M3 screw with a washer through all the eyelets and add a square nut. Tighten the nut and wrap the whole lot in tape. This method was popular in Austria until the early 1960s and I've seen it fail time and again. I don't think I've seen one single house with that connection method that didn't have multiple loose splices.
