What exactly is meant by this. Some vehicles? Which ones. What drilling? Please be specific because I am not going to drill my 2010 Prius V.
On the Gen II additional holes were/are needed too. By being clever, it is possible to mount the mud guards/flaps without jacking the car or removing the wheels. Here's a pretty good thread on mounting mud guards, pictures and everything.
Also note that the drilling is in the plastic rear bumper edge - not any metal. It isn't much drilling either - you could achieve the same effect with an awl, but drilling is neater. It really isn't a big deal.
All the drilling is under the car where you will never see them. I got the mud flaps from the UK dealer a while ago and the install was every simple.