Lack of built-in flash on OM-D annoying or frustrating?

Eric C wrote:

Thinking of selling my G3 and moving towards OM-D for following reasons

1) OM-D better camera overall

2) higher FPS rate

3) image stabilization

4) better white balance (IMO the G3 is horrible at this)

As a result losing 2 things

1) Swivel screen that rotates towards front

2) Built In Flash

The lack of flash is what concerns me more; I know it comes with a snap-on attachment; those of you who have one do you just carry it around in the pocket or leave it attached? I wonder why Oly didn't consider building it into the camera like most SLRs do nowadays, the Rebel series for example.