There are several phases to a disaster - planning, response, recovery, mitigation. During the response phase (initial response to the disaster) one of the biggest needs is for organized manpower - in Haiti, for example, to just move rubble and rescue people (In massive earthquakes, 98% of the victims who were trapped and survived, were rescued by other survivors, within the first 12 hours post disaster). In floods, the need is for people to fill sandbags, to evacuate threatened people, etc.

Thats the kind of stuff militias can do well. Unfortunately, until the first excellent response, there probably aren't many jurisdictions that will include them in the planning phase (it's tough enough to get volunteer groups of any sort taken seriously).

The recovery phase lasts for months (or years in the case of Katrina) and just the time frame works against volunteer groups. Mitigation is trying to fix the problem so it doesn't happen again (like fixing the dikes in NoLa).

But, during that response phase, there is a lot of good will that can be generated by doing some good work. Don't discount it. And since Militias are the local people, they may know where to find that part to restore the water pump, or fuel for the generator, or whatever while an 'urgent' request from yet another town or village through channels to FEMA or the State just gets lost in the shuffle.