Why is it necessary to re-circularize a plasmid prior to transformation into cells?

It is necessary to use a plasmid instead of linear DNA in transformation (into bacteria) because some bacteria such as E. coli will recognize linear DNA as invading bacteriophage DNA and degrade it instead of being taken up. The bacteria that are naturally competent can actually use linear DNA by degrading one strand and adding the other strand to their genomic DNA.