One reason to use command-line methods to access e-mail hasn't been mentioned. With shell-level e-mail commands, you can use shell scripts to automate e-mail actions. Spammers do this kind of thing, but I'm talking about legitimate uses. I've written a shell script, for example, that takes the results of a database search, converts it to a spreadsheet CSV file, and e-mails it to the person who requested it. With GUI e-mail apps, AppleScript can probably be used to do the equivalent, but shell scripts and command-line apps have the advantage that they can likely run on other Unix platforms too.