Yes, I think mass marches can be useful-- although fewer and more strategic would be better.
If you look at the MMM website and other commentary, a lot of folks are quite self-critical on the fact that there wasn't enough community building and followup out of the first march. So there is a lot of emphasis on not wasting the fact that so many people are coming together for the march and using it to deepen institution-building around the MMM goals.
Doug said that despite giving his name to ANSWER, no one had tried to recruit him. Which is the problem; people attending such marches should be recruited for followup organizing in their communities in a very systematic manner. The problem is that ANSWER is so oriented to their march and only march strategies that they collect all these names, but don't redirect those efforts to other local organizing-- since they have no local organizing plan.
-- Nathan Newman