I usually like to sketch in ink to get my thoughts out and onto paper. Working with ink seems to force me to either make a fast drawing or to add layers of cross-hatching to bring out detail. Ink is not very forgiving and so it's not hard to over do it. Some times knowing when to stop can be the most difficult thing when working with ink.
Most artists sketch with pencil because they can erase mistakes and rework the image. I have found that most great images or ideas come from places where you least expect them. If I erase a “mistake”, that image is lost forever for learning or use in the future. Being able to look back at past sketch books for ideas or to see how my work has progressed is an indispensable tool for creation. In the end, I have good and bad ideas, all of which are useful and nothing in a sense goes to waste.