As an observation, regardless of how the aids are set up / associated and turned on or off by ATC, don't forget that failures on the aircraft can deprive the pilot of one or more elements.
e.g. since an ILS/DME is 3 receivers on the aircraft, you could have a failure of the localiser and still have glideslope and DME indications.
Wierdly enough, this happened to me recently. The Nav control box glitched and succesfully tuned the Glide and assocated dme, but instead of the localiser left the nav receiver tuned to the previous frequency, which happened to be a VOR just north of the field.
Just proves the point of the importance of identing aids.
Keithl,
For an interesting exercise, try this. Find somewhere with an NDB and an NDB/DME approach, which have different minima. (generally this will involve a stepdown fix that is related to the NDB only minima, to get you past a limiting obstacle). Have the student fly an ndb/dme approach. Once they are past the final stepdown fix (i.e. they are now below NDB only minima) fail the dme. Most people will go around, but in fact it is quite reasonable to continue to the original minima. Incidentally, this is not a silly exercise - it actually hapened to me - on my initial IRT !!
CPB