It really boils down to the actions of the consultant.
I've seen some who are in it to run up a bill, often networking their buddies for a piece of the pigeon's wallet.
On the other hand, I know some really gifted people who can come in, straighten some things out, and teach the staff how to manage the problems for themselves, and get out.
Personally, I look at the best consultants as being the ones that can come in and fix something that broke, tell you what happened to make it break, then explain how to keep it from breaking again, and then leave without you being broke.