In complaining about broadcasters — an easy sport, but an irresistible one — I deplored their tendency to pronounce the t in often. Jan Freeman took up the point in her excellent blog, Throw Grammar From the Train.
Two points are indisputable, and I bow to Ms. Freeman: Offen was the dominant traditional pronunciation for centuries, but sounding the t became common in the twentieth century. Both pronunciations are current among educated speakers.
My speculation, based on personal observation, is that the t was sounded by middle-class people concerned with appearing educated, and, middle-class people with status anxiety being numerous, they gradually made the previously scorned pronunciation commonplace.
Further illustration of Bernard Shaw’s point that everyone is judgmental about spoken language comes from comments on Ms. Freeman’s post:
From Larry Larson: “I say the ‘t’. And I am a grammar Nazi. Am I wrong? I don't think so. Older American dictionaries might I find the OFF-en pronunciation throws people into the same speaking category as those who say ‘libary’ and ‘seprit’. But that's just me.”
And from someone wisely choosing to remain anonymous: “OFF-ten has always been the traditional educated American pronunciation. Offen is a pronunciation for people who warsh their clothes in the crick and write with a pin.”
Perhaps we can discuss another time why grammar Nazi is not a term to display with pride. And as far as Anonymous is concerned, the commonly accepted Rules of Disparagement in American culture allow me to make remarks about people from Appalachia, but you cannot unless you also grew up there.
As the point of what has been the traditional educated pronunciation, I refer you to the first edition of Fowler’s Modern English Usage: “The sounding of the t, which as the OED says is ‘not recognized by the dictionaries’, is practised by two oddly consorted classes—the academic speakers who affect a more precise enunciation than their neighbours ... & the uneasy half-literates who like to prove that they can spell. ...”