It takes skill to draw and when you skip that and just trace something you're only doing half the art.
That aside from the theft and moral issues involved.

if you're tracing parts of your own practice sketches from beforehand to make a more finished work then thats ok. if you're tracing from art-teaching books for learning purposes and then go from there to drawing the stuff on your own and developing your own style, thats ok. but dont just keep tracing and not doing anything on your own.
tracing parts from a book or someone else's work for your finished work, if that's what the assignment was, fine. but for regular work i kinda just go "meh" at it. (example: one makes an "OC" of a sailor moon character by tracing or using the original lineart from a coloring page and just using different colors and adding in more lace or bows, etc.) i find it kind of dull. i used to trace heavy metal cartoons for fun cuz they looked so cool. i was able to draw a few on my own but they were the less-complicated ones. i submitted them at first, but then realized how strange they looked. so i deleted them and then submitted my own fantasy drawings(under my own style which i've developed since grade 4). i like them much better and they dont look as grotesque as the traced ones.

Depends on what we mean by 'tracing' I suppose. If you mean using other people's photos to create the base of your own drawings/paintings directly copying the image, then yeah it's bad. But that's less to do with the actual 'tracing' and more to do with stealing the original work and passing it off as your own (not much different than taking someone's work and photoshopping it slightly and claiming it as your own creation). But as for tracing as a tool? Well, many renaissance painters and on through history used image projection and other forms of tracing to do the initial stages of drawing in preparation for a painting. If it is all your own compositions and stagings of photos/projections, then it honestly doesn't bother me as that is just another tool. It's only when it requires you to take someone else's work to do it that it's crossing the moral line, in my opinion. So basically, if you are tracing your own composition and photos, then I don't see a problem whatsoever. Tracing itself isn't wrong, it's the stealing of other's hard work that is.