I am working on a home as we speak that was your normal pier and beam foundation. They, homeowners wanted to cover up the space between the foundation and the actual ground. They did have good intentions, but the consequences of this action really messed up their flooring and joists. They did also have ponding of water under the home that gathered long after normal rains. After time, the wood joists, flooring, and everything began to rot. Over time it is a real mess.
Where they messed up was not to have ventilation and good cross ventilation going on under the foundation. Remember it is peir and beam. So, if you do the skirting make sure that it has ventilation.
Even if you have to carefully get under there with a candle or cigarette lighter and look for bent flame, then do that, but be careful. you need a breeze going on.