The less popular answer is programmers like to pretend their software is going to be re-used the world over, when infact the majority of projects will be maintained by a small amount of people and however nice interface-related soundbites are, you're deluding yourself.
Architecture Astronauts. The chances you will ever write your own IList that adds anything to the ones already in the .NET framework are so remote that it's theoretical jelly tots reserved for "best practices".
Obviously if you are being asked which you use in an interview, you say IList, smile, and both look pleased at yourselves for being so clever. Or for a public facing API, IList. Hopefully you get my point.
