Should I use a bottom casing Layer?

There is a very popular debate in the OMC (online mushroom community) about whether or not to add a bottom casing layer to a casing (usually a vermiculite or sometimes coir layer, notwithstanding what you used to case the top layer with). TMC refers to a bottom casing layer only once: "As an option, a layer of partially moistened vermiculite can be placed along the bottom of the trayto absorb excess water. (TMC p 134, Emphasis added). Some of the arguments for each are as follows:

No bottom casing layer:
1.) Bottom casing layers can harbor contamination that is difficult to see and detect until too late. (This is the major argument against bottom casing layers).
2.) It is superfluous and also adds more hassle than its worth. Casings fruit as well without them and are extra work.
3.) A bottom casing layer takes up too much additional room for not enough return. Space within a casing container is at a premium (since the thicker the substrate layer=the larger the flushes), and it is wasted with the addition of the bottom layer.

Bottom casing layer:
1.) The bottom casing layer acts as an additional reservoir of water from which the substrate can be supported (as well as the top layer). Since mushrooms are 90% water, the more additional moisture (up to a point where it does not soak the substrate) will produce heartier and happier flushes. Bottom casing layers can also be supplemented with additional water after the 1st flush, when casings tend to Pull away from the sides of the container. Water can be added through this small shrinkage, helping the 2nd flush along.
2.) The bottom casing layer also can act as a "run -off" where excess water and mycelial waste (mycelia produce a yellowish liquid waste when colonizing and fruiting) collect.
3.) The bottom casing layer acts as a light barrier - so that casings don't pin on the bottom. Pinning on the bottom can be a problem - if mushrooms grow at the bottom of a casing, it is difficult to remove them without disturbing the casing. But pins can rot, opening up a new vector of contamination.

Some people don't use a "bottom casing layer" per se - instead they layer the bottom of the casing container with moistened perlite. This acts to "raise" the substrate and act as a shield against any runoff or mycelial waste which may accumulate there. Adding perlite can also raise the RH (relative humidity) as it wicks water into the substrate.