Thank you Google! In the past, users had to select which address to use if the "default" should not be the sender. The process simply didn't feel natural when replying to messages originally sent to email@example.com. By default, it would set the "from" address to "firstname.lastname@example.org" rather than the more logical "email@example.com" -- even if you have the appropriate accounts set up.
The default method works well for people who do not forward messages to Gmail, but when using multiple addresses through this service it is quite annoying (and easy to forget) to change the "sender" before hitting send.
Within your accounts tab, you are now able to select the option which fixes this problem. It is not enabled by default like it should be, but I'll take it.