Xin Yun wanted to find the Five Rats mainly because he valued their unique skills, which weren't very useful in combat but exceptionally handy for stealing, scouting, and gathering intelligence. Imagine how marvelous it would be when these five guys silently crept into a warehouse, and conjured the Chaos Cauldron to swallow up all the metals! While they were improving their status within the gang, the fragments of Xin Yun's Sky-opening Axe would be gradually restored, achieving two ends with one means.
However, forcing matters wouldn't do any good, as everyone likes money, including the Five Rats. Only by enticing them with something more attractive than money could they possibly forsake wealth and trade with Xin Yun for it.