You may think you're discussing something in a calm, logical way -- but to the other person whom you're criticizing, all they see is a point-by-point attack on their work. All it takes is a couple little places where the meaning could be misconstrued, and suddenly they can become very defensive.
Yes, but ultimately that's their problem. Unless they are your boss, of course, in which case it's your problem AND the companies problem.
Whilst it's true that writing an email and then trashing it, or waiting a day to send it, is often a useful strategy (and I've done that myself a bunch of times) ..... it rarely solves anything long term. The underlying reason for the anger will still be there. People who deflect attention from their mistakes by saying "he/she offended me" don't actually improve the workplace, they just push the pain off onto another day (and make it worse next time).
The problem with email is not that it's an impersonal medium where it's hard to calibrate your tone. The problem is the opposite - when face to face with someone who is visibly angry or upset it's very hard to speak plainly and tell it like it is. People chicken out, they bail when they see the signs of anger or hurt on the other persons face. Nobody likes to be a meanie.
But now think about all the completely nonsensical, illogical, vastly disruptive rubbish that organisations engage in all over the world, every day. Someone, somewhere in that organisation knows that what they're doing is bad for the customer, or is a huge waste of money, but cannot say it without hurting someone's feelings. So things just carry on. People who really shouldn't be in the job are left alone or even promoted, and the people who can see plainly how messed up things are leave, making the problem worse. How many times have you heard people complain about a corporate culture of ass kissing?
We internalise this sort of thing as "big companies are slow, only startups can innovate" and other such business aphorisms, but often all it boils down to is that without a medium like email, it's often too hard to speak plainly and lay out harsh truths. Small companies don't have enough of a financial cushion to place being nice to your colleague above factual reality. Big companies do. But with email, natural instincts like "oh shit he's going to whack me" or "this woman is crying and probably won't mate with me now" get buried. So people find it easier to say things that might be hurtful, but might also be true and ultimately better long term for the organisation.
Of course, the line is very blurry. There are sometimes people who aren't speaking hard truths, they're just genuinely assholes. But my experience has been that genuine assholes are rare, whereas everyone can just have a bad day or get irritated at poor behaviour by someone else.