There is plenty to be concerned about with possible infection from a soiled diaper that has not been changed for several hours. Please see a discussion on this in our Diaper Primer (just click on the homepage).
In short, the body is disposing of bacteria-filled matter through fecal discharge. These will multiply and possibly cause infection that will be very difificult to stop without medical intervention.
So in addition to the discomfort of wet and soiled pants - or the very unpleasant odours - remember if you can detect them, then so can anyone near you - one risks infection over and above diaper rash.
In short, frequent changing - and as soon as possible after a fecal discharge - is both a clean and healthy practice.
