When I was in the fifth grade, my teacher taught our class how to write a business letter. We practiced again in the sixth, seventh, and eighth grades. In my freshman year of high school, we wrote model letters and notes of all types: casual correspondence, notes of thanks, letters of condolence, and business letters. We were taught the form and also the content. I can remember our teachers stressing to us throughout, at every level, that a business letter should always be professional in tone and that if you can't say something nice about someone in a letter of reference, you should say nothing at all, e.g.: "My personal familiarity with this individual is limited, so I cannot provide a reference," or simply, "I do not feel that I can evaluate this person".
Over the years, I have read everything from the aforementioned statements of inability to evaluate to "damned by faint praise" all the way up through "poison-pen letters". So often when one devolves to vitriol, the point is lost in subjectivity- in other words, the axe is being so thoroughly ground that the original purpose of the letter is altogether obscured by hatred and bad manners. Why can't the authors of the lattermost understand that?
If one gains a reputation for submitting nasty missives on a routine basis, it's not the person about whom the letter was written who will be questioned- it's the writer. People will wonder: "Is (s)he always so unprofessional? Can I trust him or her?" The answers to that are, in order, 'yes' and 'no'. You've got to wonder after a while why the writer is jealous, angry, or unhappy- these are the three things that typically engender this sort of elegiac hatred. Regard for them, and their opinions or expertise, becomes diminished.
Letters of reference should never furnish a canvas for one's polemics. They should not reflect a lack of professionalism, backbiting departmental politics, or childishness. Using them for this is tasteless, or in the finest Southern tradition, just plain tacky. Civility may be dying, but letters of reference are not a gladiatorial arena for its death.