As we know CSS copy protection on DVD has been compromised soon after the method was announced. A Norwegian kid cracked the copy protection scheme in 1999. Since then it is well known that is someone wants to copy a DVD there is always a way. The only solution is to make it less convenient, and using DVD-9 is among one and the simplest one.
The argument is that dual layer DVD-R is more expensive and is not very convenient to copy. Most disc copying software, including the famous NERO, is not very well equipped to do dual-layer copying. Layer break point is linchpin . Most software simply cannot figure out where the layer break point should be and will do a simple fill up the first layer and then dump the remaining data to the second layer scheme. Doing so will definitely create a coaster.
Do consider using DVD-9 if you want to make plagiarism more difficult.