I am in the process of replacing my battalions with MCVs (just like I replaced my water tankers with pumpers). I noticed in my ARRs that if I require a water tanker, a pumper will be deployed. So that's helpful. Similar to if I need to send out a patrol, a sheriff will be deployed.
But, if I need a battalion out, it wont send out an MCV. Even though a MCV can fulfill the role of a battalion.