I differentiate between two kinds of bad English:

(1) Done on purpose for some reason (to keep a rhyme, for instance)

(2) Done because the person doesn't know how to do it right.

Items in 2 are always irritating. Items in 1 can be merely quaint (like "swungen/sungen") or downright grating (like "words she brang to me"). I imagine the line between the two categories are fuzzy and different for each person.

Alex