I think a disposable broom would be useful in the instance you are cleaning something up that you would make you not want to use your broom again after the mess up (i.e. vomit with the sawdust stuff on it - it happened today in my classroom).  Just threw the broom away!  If there was a disposable broom for this or oil spills sopped up with cat litter on them, etc. then it could be used to clean the mess up and not contaminate your regular broom.