Tim,
That's a good question. We've looked at buying a MM several times and still haven't for some reason. I'd be curious to hear from those who have and use them for timbers -- I couldn't find anyone with directly relevant experience when I was looking.
The reason we would get one is if the specifications on a high end job called for the timber to be below a certain number -- 19% is the one I see in specs most often. In that case, it would be helpful to be able to verify that my supplier is meeting spec and that my product after fabrication is meeting spec.
DL is right that equilibrium moisture content varies regionally. Here in the humid southeastern US, I consider 12% to be EQMC.
One thing to note is that there are two types of moisture in wood -- bound water and free water. the free water is the water that exists outside of the wood cells. The free water is the first to evaproate. For most species of wood, once the free water is gone and all that is left is bound water you are at around 30%MC. It's important to note that this is the point at which shrinkage begins -- when the cells begin losing the bound water. It continues until equilibrium MC, which is dependent not on relative humidity but absolute humidity levels as DL said.
(As an aside and not to encourage a green vs dry debate, I believe there is ample historic evidence that many high-end buildings were built with dry wood. I'm talking churches and great halls, not average residential buildings. In our work the same is true -- high spec/high end projects usually use the more expensive dry material while "normal" jobs use green.)
