EMAC has been used repeatedly for floods, hurricanes, and fires. Procedures have been worked out in a variety of situations. The fact that procedures are developed, written, and tested in actual disasters is a strength or selling point for EMAC.
EMAC uses nationally-accepted standards. A wildland strike team has the same definition in Florida as it does in Montana.
EMAC is legislatively approved by both Congress and the States. It is very unlikely that states will opt in and out of EMAC with each change of administration.
Finally, an advantage for EMD and each member state is in opportunities for training. Should Washington State be in the enviable position of not having any disasters for a lengthy period of time, EMD staff skills could be maintained by assisting other states when they have disasters.