Ok, I'll step in with the fire extinguisher here. There are valid points on both sides.
In general I never recommend any type of installation where the factory wiring is cut to remove the OEM connectors and wire the lights in directly. This makes going back to stock (or even removing the lights for service or other reasons) more difficult and can diminish the value of the car at resale time. However, ultimately it's your car and you can do what you want with it. If you do choose to do a "cut and play" install at least make sure that all the connections are mechanically and electrically sound as well as fully insulated and sealed.
Aftermarket connectors are available if you want to make your own harness or wire the connectors directly to the lights themselves so they can then plug in to the factory plugs with no cutting. While the quality of said connectors may not be equal to factory spec ones they are sufficient as long as the above mentioned precautions are taken. Unfortunately the average person doesn't have access to the original connectors so while this is a nice way to go it isn't a viable option for most do-it-yourselfers.
So in short, the most important thing to bear in mind with these installs is to employ proper wiring methods.