[educationdata]

Very good point, but only half the story. Marker has another problem: eraser. Most of the time the eraser is already too dirty to use.

[brianpgordon]

But a chalkboard eraser just smears the chalk around. Even with a decently clean eraser, after a few write/erase cycles you can barely see anything because there's so little contrast. Dry eraser marker goes on vivid and sharp every time, and with a high-quality whiteboard will wipe right off cleanly.

[taejo]

When I TA'ed in Germany, every classroom had a sink, a sponge and a squeegee, so you actually cleaned the board every time instead of smearing it. It was great.