The main reason why parasites and imposters are brought up is because many dive into evocation for the first time without developing senses, methods of protection, or even banishment. By bringing it up and teaching methods to prevent them through our tutorials, that possiblity can be eliminated and we can get into the meat of the subject. The downside to that is problem you have mentioned, where any negative experience has the parasite label slapped on without eliminating all possiblities.
Any spirit (god, demon, angel, whatever) can attack and be nasty, for whatever reason. It is foolish to believe all will be nice to oneself (for example, Alastor is not exactly known for being kind whether you are looking at his Christian or Greek lore). Whatever a spirit can be called upon for can easily be done to the magician. Hence why it is important to protect the self.
At the same time, parasites and imposters (as it is easy to shift the outer parts of your energtic body and make a claim to be someone else) do exist as well, so labeling all negative spiritual experiences stemming from the popular spirits is also unwise. Ultimately, both scenarios can be dealt with through the same methods. The main way to tell the difference would be to 1) use the techniques meant to prevent parasites and imposters and 2) repeat the evocation to see if you get a similiar result. If the result from the repeating evocation is pretty much the same, you know that is not a spirit to be worked with.