Without any political connotation, of just email as a communications platform: Email incurs costs of mental kind on the end user. I see nothing bad of not using it - if one can afford not to. For example, Donald Knuth seldom uses email as well. http://www-cs-faculty.stanford.edu/~uno/email.html
I'm not sure if it's such a great strategy for a future president, though (especually as he seems to spend awfull lot of time on twitter).