Well there are logical explanations to these demons, hearsay as Brysing said being a major factor in a lot of mythology. Many demons like succubis/inccubis could be explained with sleeping problems [tightness in the chest, seeing ghostly shadows floating around the room, and being unable to move are symptoms of sleep paralysis]
Anyway, for argument let's say they're real, the reason so many want to summon them is because most are naive teens who think 'something I can have mind blowing sex with? Sign me up!' When they are dangerous, being known to drain a victims energy until they die, but most who summon succubi/inccubi don't research or weight the good and bad, like most things they want results now, and meeting someone, going out on dates, getting to know them and so on is hard, saying some words and something appears to fulfill your needs is easy.
If these demons are real and someone is foolish enough to call on them, then they will learn their lesson the hard way that the easy route isn't the best choice. If anyone does wish to summon one, study and know what you're doing, that's all you can hope for really.