Being able to play .ISOs would be nice -- but I have to say, for me personally, it's not a big deal.
For most DVD movies, the only part of the DVD I care about is the movie itself -- I'd just as soon not bother wasting drive space or network bandwidth on menus, trailers, commentaries, languages I don't speak, and so forth.
So I've just stored the main feature of all my movie DVDs as MPEG2 files -- not transcoded, but the DVD streams copied into an MPEG-PS container. They're big, but still a good bit smaller than the .ISOs because all the extraneous stuff is removed, and it's still a true copy of the DVD's audio and video streams.
I guess the point in playing straight from the .ISO would be if you wanted DVD menus and chapter selection and such -- but I think storing all that stuff would just be a waste of space.
Or have I misunderstood why people want this feature?