How can this be potentially hazardous?
Some junk-mail use exploits of the Outlook browser to execute their files without permission, and to delete said junk-mail usually requires selecting it and hitting the delete key, thing is though, just clicking on it once brings it up in the preview pane, which, if it does indeed contain an exploit (disabling HTML is one thing, but there are always other exploits, like the jpg buffer overflow that was patched quite awhile ago), will shoot off and infect yer PC right off the bat.
[EDIT] You know, I suck at explaining crap, maybe someone else with a better grasp of the english dictionary can clarify things.