For me the main reason I would choose Emacs over an IDE is because it allows me to do everything from just my keyboard. This is nice in that it saves some time for when I would normally use a mouse. Also since I find myself very mobile I have a tendency of having my programming "groove" interrupted due to using my slow touch pad. In addition its customization makes it shine over some IDEs for me. However if you find yourself programming fast enough with an IDE then I would say the learning curve of Emacs is not worth the trouble.
