I'm going to sit on the fence here and say that I can agree with both sides of the argument...
For everyday, regular road use, where you are not towing a reasonable sized trailer, then I would agree that a 4x2 car is perfectly adequate.
However, if we are saying that you need to tow maybe 1500kg or more, then a 4x4 comes into it's own. They are not only far less likely to loose traction whilst pulling up hill (remember, the trailer nose weight will always lift the front axle slightly, so reducing grip), but when using engine braking on a steep decent, the effort is fed through all four wheels rather than just two, again making the whole experience far more sure-footed.
My Discovery (which is permanent 4x4) feels like it is 'clamped' to the road, even with 3000kg+ on the back - and that's a great feeling when you come over Haldon Hill with the rain beating down and the wind blowing
However, 4x2 or 4x4, I feel the biggest cause of towing accidents is excess speed, followed closely by overweight or incorrectly loaded trailers, then poor or non-existent maintenance