You may be able to accomplish what you want by manually editing the tag values in the timer XML file, setting to 1 the specific days of the week and the holiday bit. I change this parameter to all bits off (0) to effectively disable a particular timer. Certainly not as elegant as being able to do it through the GUI but if it works would avoid the need for duplicate timer definitions.
I would add to this request the ability to set all check boxes off so that a timer can be disabled. Perhaps removing all the GUI logical validation and letting the user decide what is most useful.