Revenue Management
H&S I totally disagree about your definition of the Revenue Management role. Therole of RM in any airline is to optimise revenue and LF according to demand and seasonality. It's all about asset utilisation.
Revenue Management doesn't mean "filling" every flight because sometimes this isnt possible. If you look at FR's promotions in detail you will notice that these are for off peak flights only (M-Thurs) and also large percentage of these promotions are run during the traditionally quieter winter season.
I admit that a no show will have a slight adverse affect on FR's LF results however on zero tax/fare promotions they will gain slightly from the debit/credit card fee. The only risk of offering zero fare tickets is incorrectly ofering these on flights that in ryanair's eyes are forecast to fill, where the seat could potentially be sold for a higher, more profitable break even fare.
The most important part is that being the astute company that Ryanair are they have probably factored into availability that 10% zero fare don't show therefore they overbook by 8% for example to mitigate the risk of flying light. A tricky task for any RM department but this is what the legacy carriers do well.