Question
I know that using a mulching mower is not supposed to cause thatch.  But I have wondered whether using a mulching mower when you already have thatch might add to the thatch build up. In other words, might the fact that a layer of thatch already exists between the soil/rootzone layer and the new clippings, mean the soil microorganisms cannot break down the new clippings?  The lawn books all say that that is not from grass clippings, but is from old stems, etc.  But certainly mowing also cuts off stem materials.

Answer
Generally, mowing does not lead to thatch. If you mow very tall grass and you already have a thatch problem, you can make it worse.

If you mow at the recommended height (2.5-3.5" for your grass types) and mow regularily so as to not cutting more than 30-40% of the length of the grass during any one mowing then you should be fine. Even on thatch, grass will decompose fast, and technically the decomposition process will provide microbes which can also break down thatch (in theory atleast).

I strongly recommend mulch mowing compared to bagging.