One of the joys of litigation is engaging in a seemingly useless campaign of writing letters around discovery issues.  You anticipate disputes over discovery.  The parties are constantly posturing.  And you want to have laid the foundation for having been reasonable when you finally take the dispute to the judge.  Part of this process (traditionally) is writing letters back and forth. 

It seems like emails could serve this purpose equally well.  There will probably be the stray dispute around whether opp. counsel actually received and read the email, but those arguments are not getting much credence in court.  Plus you look pretty silly making them.  So will "correspondence with opposing counsel" shift to email?  One other argument is that the unstated code that email is reserved for casual/minor issues, while a letter signifies gravity.  I'm not so sure.  People tend to respond to email quicker. 
 
NB:  A loosely related post on the death of voicemail (or the primacy of text over voice) here ("The Death of Voicemail in Law Firm Marketing").  I'd very much agree with that post, I don't exactly fit into that demographic but I strongly prefer text to voice.  I can't let my voicemails pile up for a couple of days, but you will surely get a quicker answer for anything if you email me.