Why keep plasmid in non-T7 RNA Polymerase strain?

With extended culture the expression level of plasmid genes in a T7 RNA polymerase-containing strain decreases. This has been found to be due to mutations on the bacterial chromosome decreasing the amount of T7 RNA polymerase produced, and thus decreasing protein yield. To get around this the plasmid can be preserved in more run-of-the-mill bacterial strains and re-transformed into a T7 RNA polymerase strain when more protein is needed.