Why do we do bacterial inductions - (Mar/03/2014 )

This is a really noobish question, but why do we have inducible constructs (e.g. IPTG induction) for the generation of large amounts of protein?  Why can't we just pop a plasmid in e. coli with a regular e. coli promoter and harvest those cells for the generation of heterologous protein?  Does it have to do with the amount of protein that can be developed with inducible constructs?

High level of protein expression reduces the viability of the cells. They grow more slowly, and can be out-competed by normal cells. By growing without expression until dense cultures form, you avoid most of these issues.