That's true the one time I got a d&c I had remaining tissue they saw through the ultrasound (transvaginal). Your body is amazing but technology is here too for times like those. I've gone both routes just don't let anyone push you one way or the other feel what you need to do. Some go natural some d&c (especially if necessary), some take a pill (vaginally I think if I remember correctly) to start contractions to make the process faster ( again preventing infection).
Sometimes it is called for, if some tissue from the placenta has remained in the body. This can be tested for by testing your hCG levels. If there is still a remaining number (mine sat at about 50 for several weeks) it suggests remaining tissue. That is not the norm, however. Most of the time, a natural miscarriage takes everything out and that is that. If your periods are normal and you have no other signs, you probably don't need anything.
No not always. The d&c is sometimes pushed and I do not see why unless that is what YOU want or are enduring severe complications (fever almost fainting) basically ER status). You will most likely need a d&c if you get these symptoms to avoid infection. Some doctors do think d&c is easier emotionally..