If you'll note my points, there's functionality above and beyond just plain display that a proper eBook app has. From what I've seen, present PDF readers lack these.

Whatever other "mobile" formats come into being, relying on a publication format which presumes a fixed, print-based output (in any of several internationally incompatible formats -- I've just learned of the US letter/A4 hybrid though I've also instantly forgotten its name) ... in a world in which text displays are overwhelmingly both electronic and fluid, strikes me as very backward.