Tater makes an interesting point as the radio side, NPR WHYY-FM does exactly as he suggests and airs their normal programming with interruptions planted into the normal shows (which are just as annoying during ATC, Morning Edition, Car Talk, etc as during a "special" on PBS). It seems to work fine for the radio side of WHYY, so it could work fine for TV.
However, there also is a point for airing specials that might pull in other donars than the "normal PBS viewer" where they'd also pony up some cash for their favorite special, Big Band ballroom dancing aimed at WW2 generation, Doowop aimed at the 50's generation, Peter Paul and Mary aimed at the 60's gen, Osmonds aimed at the 70's gen, etc. My guess is, PBS assumes that their normal viewers will support their favorite shows so they can continue to be shown and understand the occasional interruption from the normal schedule as the price for commercial free quality televison to encourage others to also support PBS.
Another way of looking at it, is these are those moments when PBS lets its hair down, sort of like on NPR when they air Car Talk. All quality shows, but not the usual sort of programming you'd expect from PBS or NPR.