I agree with this, although its not quite as easy as you might think, as not everyone acknowledges holidays as a non work day.
The only way I can see this being usefull is to add an option that allows you to select all the holidays that your company acknowledges or wants to include in a 'days that are not workdays'.
I'm sure there's some algorithm to figure out what day is Easter, etc... each year, but otherwise I might be inclined to think that an update every year would be required to maintain the dates for each potential holiday that year.
I know other scheduling software has provisions for holidays (our old Product, Tidal Enterprise Scheduler was one of them.. $$$$) and i'd love to see it. Alot of thought needs to go into how it'd be implemented though :)
Brian