Bad Boston Accents

But... Ben Affleck??

~ I'm a farmer, who's ever heard of a fatalistic farmer? ~

I know. The actress playing Ben Affleck's wife had a terrible accent.

Ben & Matt have succeeded in creating this wierd image of Boston. True we don't all talk like that.

Obviously, Ben Affleck is speaking in a manner that's very close to (if not identical to) the way he naturally talks, so that covers him (not that the OP was complaining about him, of course).

My observation: the accent was used as a social-class marker. The Costner character (and the wife) were supposed to be from a working-class background. So far as I know (which isn't all that far), the accent still has class connotations.

For a very similar, and even more obvious, use of accents, see "Good Will Hunting." Affleck (at least to my perception) cranked the accent up a few notches, Robin Williams affected it, and various other characters either put it on or exaggerated it. The casting of Stellan Skarsgaard, Minnie Driver and George Plimpton created huge accent distinctions, but even when more subtle it helped put various scenes across (e.g. the confrontation with the Harvard students in the bar: "my boy is wicked smaht").

Since you mentioned Alec Baldwin: another rather obvious instance of using the Boston accent as a class marker occurs in "30 Rock," when Baldwin sometimes slips into it ... and, notably (comically notably), in Julianne Moore's character.

Whether Costner actually got the accent right is, of course, a different question.

Actually Kennedy's accent is more of an accent from bygone era. This used to be the prevalent Boston accent in the 19th century. They didn't develop it. They just kept it for some reason.

The entertainment industry, especially movie making, lives off of stereotypes. People who make these movies live either in the LA regions or the NYC region. When they think of accents they think of the most popular image of that region and then hire a dialect coach to train the actors to speak like that given popular image, even if that image is true to a very small percent of a given population & region. Of course there's the person that always interjects and say "I live such and such place and it's true! People do talk/dress/act that way!" Usually these are self-loathing dolts.

2014: Whiplash, Cold in July, that Terrence Malick project set in Austin

Agreed, but nothing is bad, and no accent more butchered, than Southern. It's rare that someone does a convincing Southern accent in the movies or on TV unless they actually grew up there--though this is probably the case with any regional accent. However, actors doing Southern always err on the side of "Jethro Clampett" when in fact "Julia Sugarbaker" is more authentic unless they're in a really rural setting.