Quote Originally Posted by Mr Peabody
You can still get a good connection with bare wire and a bit of solder to coat the bare ends helps prevent oxidation.
The benefit of using bare wire is it's malleability. When compressed, such as when tightening a screw or inserting into a push-to-insert connector, it is compressed and makes a tighter, more secure connection that also spans a slightly larger contact area.

Tinning the ends, while looking cleaner, takes away this ability and makes for a less secure connection.