I have always made it a point to respond promptly to emails that require replies. Sadly, not many people make the effort to reply to emails, even though reminders are sent or when there is a “cry for help.”
Responding to letters and emails helps to improve communication and builds healthy relationships. If someone makes the effort to sit down and write an email, it is only gracious that we respond to him/her. With technology, emails can be crafted and sent out within seconds, so there is no excuse not to respond to letters sent via this mode.