A lot of things have changed with the increase of smartphone users, especially for website owners, email marketeers and designers. Whereas before it was okay to design with one size in mind, nowadays you will need to ensure compatibility for the complete range of mobile devices, tablets, and e-readers.
This infographic provides some background and statistics on why this is a trend that should not be ignored and at the same time tries to stipulate the importance of responsive design for the near future.