[Brian]

Sorry for the confusion here: iCal won't accept to-dos without assigning them to a calendar. iCal's calendars are mapped to contexts in OmniFocus.

Mapping calendars to projects means you would have to set this up individually for each project.

For most folks that use OmniFocus, projects come and go, but the contexts they use tend to stick around longer. (A "work" context lasts longer than "Prepare for meeting", in other words.)

Mapping calendars to contexts saves effort in most workflows, but it does mean that you'll need to set up at least one context. (If you just use it to hold the things you want to sync to iCal, you can leave it blank for everything else.)