Such spirits darken the host. Worse, most people can't tell they are carrying such things and don't notice the effects. Even those with psychic abilities often can't find all the hitchhikers attached to themselves. Dark ones lie . They do not keep bargains , even though they'll try to force you to keep your end of a bargain.
Spiritual fighting is hard to do alone, and the consequences of losing are dire.
They're difficult to detect, and you can't often tell what they're doing. Another possible, worse problem that a friend of mine has observered, is to lose so much of one's energy to attackers that one becomes completely apathetic. The person being attacked may not even be aware that his or her energy is being drained away like that, aside from internal bad feelings that result from symptoms such as hypocrisy. The darker you are, the more you tend to hurt others. Think also of the grief of those who care about the person, as they watch what happens.