It's worse when they turn a lassie into a Lassie. ;-}

My dictionary says "ain't" derives from "am not", was widespread in the 18th century, and is still perfectly normal in many dialects and informal contexts in both North America and Britain.

I only find grammar "errors" unacceptable when they violate the conventions for the dialect of the characters (including the "narrator") in the song.

If I recall rightly, mg asserted that rhyme is more important than grammar, but I disagree: a lapse in rhyme or a "poor" rhyme is quite common, and is seldom given much thought, whereas a grammar error (which does not accord with dialect usage) will cause the audience to wince, and that more than nullifies any benefit gained by reaching for the rhyme.

Moreover, if a songwriter is serious about his craft, he guards against becoming overly attached to what he writes. A stand-up comedian is expected to write at least five times (and closer to ten times) the material he will eventually use--any less, and it's rare that he won't bomb. He must resign himself that 80%, 90% or more will just have to be trashed--and that's before even facing an audience. Perhaps if songwriters wrote more like comedy writers, the label "singer-songwriter" wouldn't cause so many people to cringe.

I raise this point because if you're having to resort to awkward grammar, maybe you should toss the tack you're taking and find a new one.

A different standard also applies to adapting old works than to writing new ones. We expect a lower level of literacy and poetic quality in old lyrics. With new works, you emulate that at your own risk. Dialect is fine, but Diamondesque lyrics?--well, you'd better be a Neil Diamond.