Of course, there can be other reasons to adjust language -eg in some songs, even if "dem" and "dese" may be accurate enough ways of writing down the the way people say/said "them" and "these", to sing it that way when it's not the way you talk can come across as patronising and contemptuous. The same goes for some grammatical and syntactical constructions.

And similar factors can come in if the grammer and syntax is more formal and correct than the singer is comfortable with - maybe when a poem is turned into a song, and singers might otherwise feel people might think they were taking the piss out of the original.