there must be a heap of these things junked the moment the exams have been passed
No way! I love using it, and do so most times I fly, unless its circuits or local area I am very familiar with! The other exception is when I just use mental DR to estimate correction angle based on max drift - but this is generally when there are two PPLs in the cockpit and the workload is reduced.
Why don't people like them? They are easy to use (if you learn!), fast, flexible (in that they can be used for a variety of problems), they help you to visualise vector problems, and they are safer in that you can - again assuming you understand how they work - readily spot gross errors (versus a calculator on which you might simply mis-key one operand or operator and hence get a wrong answer).
Anyway, I suppose the question wasn't whether I like using nav computers and why, but was rather whether people do use them after training: I do at least
Andy