Hey Micro, here's my take on this (... yet another guy down the pub says?)
Inside a perfectly sealed jar of herb you have air, herb, and water. The water will distribute itself between the herb and the air to give an equilibrium where the moisture content of both the air and herb will stay constant (I think that this will depend on temperature in a perfectly sealed jar, but I'm not totally sure). If you also add a Boveda pack then this will control the humidity in the air, and hence also the moisture content of your herbs.
If you open the jar then you could exchange your moisture laden air from inside the jar for dry ambient air, and if you do so once the jar is closed again the herbs (or Boveda pack) will need to release some of their moisture in order to restore the equilibrium which is appropriate to the storage temperature. If you don't use Boveda (or something else to restore moisture) then your herbs will be slightly drier afterwards (although perhaps not noticeably). Over time the situation will get gradually worse as the jar is opened.
If you're using Boveda then it should compensate, but only until it dries out too far. Ways to combat the loss of moisture would be to keep the jars as full as possible, and to minimise the number of times that the jar is opened. The problem would most likely get worse more quickly as the jars get emptier, as there is less herb to supply the required moisture, but also more air requiring moisture to be supplied. As I see it the purpose of the Boveda is to provide a controlled source of moisture to replenish that which is lost every time you open the jar to get some herbs out.
I'm in awe of your restraint though, long term storage isn't a problem for me because there's nothing left to store by then.