Another great use for onboard flash is when you're travelling - I recently had a holiday in the US where I took my 20D everywhere and I used onboard flash a lot.
If I owned a Canon 5D or 1D series that don't have a built in flash would I carry around an external flash unit - no way!! I would leave them at home and pack my 20D.
Onboard flash = win
If I owned a Canon 5D or 1D series that don't have a built in flash would I carry around an external flash unit - no way!! I would leave them at home and pack my 20D.
Onboard flash = win