The "problems" that Hellbore is having with the Disengage rules are all very realistic ones and IMHO, not really problems at all.
I have done several martial arts (including S.C.A. and rapier fighting) and studied warfare tactics, and from all that I have been taught and experienced, turning tail and running is a very bad thing 99% of the time. You will get smacked in the back.
In Dark Heresy, there are options such as Knock-Down that will make for a safer full-move retreat. As others have pointed out, Disengage is very useful in the right circumstances, and realisticaly, turning and running full out will only get you smacked (which is how the rules work).