Are there benefits in training instrument students how to fly a DME arc?
Apart from the fact that it is required to learn, the obvious "benefit" is that you know what to do when flying to an airport that uses a DME Arc approach. RNAV/GPS approaches are becoming more common, but have far from taken over from conventional approaches. Even if an airport has radar, it does not necessarily mean that radar vectoring for final approach course is available - consider, for example, uncontrolled airports. Here is an example of a DME Arc approach still in use today, at an uncontrolled airport in Denmark that does have radar, but no radar vectors are provided (since it is uncontrolled):



And an example from Norway:



(images from AIP Denmark and AIP Norway respectively)