David Sparks in why he doesn't mix calendar events with OmniFocus tasks:
Put simply, tasks are things I need to do and calendar items are places I need to be.
Exactly why I don't sync OmniFocus with iCal. Each serves a very different purpose. I do, however, block time in my calendar under "Do Stuff" when I definitely need to go through some OmniFocus tasks and I cannot afford to be interrupted.