A couple of progs will parse an ISO internally to provide support for "direct" DVD Playback but as a big ISO format supporter I personal have no desire for MC to try to do this for a couple of reasons:
1) ISO is an image container designed to hold all the info on a physical disc. This disc can be a CD, DVD, Blu-ray, HD-DVD and who knows what else.
2) Existing Mounting Progs like VCD do a great job of parsing these ISO files and mounting them correctly so that the become available to Windows/MC as if they were a native CD, DVD, Blu-ray, HD-DVD and based on the Autoplayback settings they will then fire up the appropriate app / playback settings.
3) MC lets you add coverart, tag, manage, launch the mounting of the ISO, and then the playback of the content if it is a DVD (and if it is a Blu-ray it plays in TMT etc).
Its taken Slysoft has for years continued to support and develop VCD and IMHO it is the best of them out there. I'd rather MC folk keep working on stuff we need elsewhere and not try to replicate stuff that already works very very well.....but they ignore my arguments on this all the time - so like anything if enough people want this feature then I'm you may be able to convince them!
Thanks
Nathan