As a student of osteopathic medicine, I believe i can bring some insight into this thread...
First of all, some of you are correct. Some chiropractors are garbage, and they can do a lot of harm. On the other hand, some actually can be quite effective.
Second,..... OMM (Osteopathic Manipulative Medicine). For those of you who aren't privy to the facts about it, I can honestly say that i've experienced it first hand, and i've seen it many times and am currently learning how to do it now. It works. It is not the end all be all cure of cancer or heart disease. It IS however, a great supplement to all of the other tools we all learn in medical school and can be quite effective in treating common ailments. Think about it... what are the two most common complaints when your average joe comes into a doctors office??? 1. "doc, my head hurts" 2. "doc, my back hurts". These are two very prominent ailments that OMM proves very effacacious in treating. It also is proven to help with more advanced ailments. It is only another alternative to offer or a supplement to other treatments for these ailments. In my opinion, the more options you have, the better.
Here's where chiropractors come in. There are some good ones, and they do practice something very similar, but not exactly like OMM. The difference is that Chiro's usually do a lot more high velocity thrusting, and this, in my opinion, and many other DO's opinions can be dangerous. The chiro's who are not trained in a more scientific sense can do a lot of damage. The fact is, however, that if every DO out there emphasized OMM a lot more, then there would be no need for chiropractors. Since, there are not enough DO's to go around... not yet anyway, chiropratic practitioners can work cooperatively with medically trained DO's and MD's, and can be quite effective if they know their limits, and have received proper training including a strong scientific foundation.
As far as chiro school being a back up for DO/MD school.... i think that if you want to be a physician, in the traditional sense of being a doctor.... then you should look into the osteopathic/allopathic route, you will be dissapointed if you have to settle for anything that isn't in that vision. If you have a modified view of that vision, then you can be very happy as a chiropractor, and many of them are, and are living very well.