Students have enough problems with CRPs which the instructors teach. There is no time to teach two techniques. The CRP is known to give the same answer as the examiners' CRPs (different from accuracy, much more important!). There is no need for any other instrument. Never have any problems rubbing HB pencil off with a finger, either!
I'm sure you are a good pilot, Tinstaafl, but have you ever lectured in use of a flight computer? I have to say that in trying to get a class of 10 people with a variety of experience from 50 hours to 4000 hours and academic background from leaving school 30 years ago with no qualifications to passing a degree last year (not always clear which is easier to teach!) then it is a lot simpler if everyone has the same equipment. This is particularly the case as there are a lot of reminders on the CRP-5 as to how to use it, which I always point out, the students use them as memory aids, and because I have a large-scale CRP-5 as a teaching aid. It is more difficult to teach if people have different versions of the conventional flight computer, impossible to teach effectively if people have radically different computers.