When does walking away from an unrewarding business venture actually indicate wisdom rather than a lack of tenacity?

Many mid-market business leaders waste their resources and talent being persistent.  Believing that persistence always pays dividends, they doggedly try to get good results from what turns out to be a poor opportunity.   This article addresses how to assess your current situation as well as hints on assessing new opportunities so you pick wisely.  Read and comment on this Forbes article.