Converting 5 unwanted materia into 1 random of another kind is by default a bad deal -- easier to sell them and buy the kind wanted via MB.
Better would be if we could convert several lower-grade materias into one of a higher tier, then the gamble might be worth it indeed.
Also, we have means to increase the spiritbond rate but no way to increase the chance for overmelding to succeed. Would be great if such existed as well.