Tim unintentionally misrepresented me a little there - I'm not into content-based courses, and my beef with quizzing is only when it's used as the main form of assessment (usually in content-based courses).
ie read all this stuff; do a quiz; here's your grade; bye!
I see a disturbing tendency towards this approach to online learning from some quarters - I think the primary reason is because it's extremely easy on a teacher - "I can sit back and just let them run through the maze I've built - they teach themselves"!
When quizzes are thought of as an activity that is just one tool in a range of possible activities, they certainly can be very useful. If the grades aren't much then cheating ceases to be an issue. I've used them in courses myself in exactly the way Don mentions. In my case they were fun things to highlight certain ideas and facts that came out in preceding forums.