That's not that easy to answer because it really depends in my opinion on what is plagiarised and why and what's the impact.
For example we submitted a paper and the publisher found a plagiat in our introduction. Turned out we had written the same sentence in one of our other papers on a very similar topic. We had around 15 papers in that field and well, the introduction is always basically the same so sooner or later you will write down the same sentence. It's just stupid to forcefully rewrite the same introduction over and over again and it has nothing to do with the actual scientific work.
And while I feel it's unethical to use previous work in university courses the even more interesting question is: why? Why would I reuse my previous work for classes at university rather than learning something new/training my skills. Why would I study, if I'm not interested in the subject.
You really should take courses at university as an opportunity to learn something, rather than an annoying obstacle on your way to your degree, which you want to avoid/manage with the lowest effort possible