using custom function 1/200 sync speed on Av mode

Was just messing with my camera and I changed a custom function to have the shutter speed go to 1/200 on Av mode. Are there any disadvantages to using this other than possibly loss of ambient lighting? I know sometimes the slower shutter speed bothers me when i do try to use Av mode. Anyway, since I never really knew about this feature, I figured I'd ask.

I don't know much about it, but it seems to me it'd pretty much turn Av into M mode, and make the flash the main light. It's pretty useless if you want to use fill flash. Hopefully someone knows something about it I don't...

Agree with Tim. Seems useless to me. Never understood it's use.

I'm not a pro, although I have fooled around with this. My explanation...It is not the same as "M" mode. In "M" mode the flash is still a fill flash, the ambiuent and subject are controlled by your settings. With this function set, in "AV" mode you're forcing the shutter speed to the high sync speed of the flash (1/250 on the 20D and 580EX). You can still control the deph of field with the "AV" setting, although the shutter will also be 1/250. With the faster shutter speed you will potentially lose bckground lighting (dark backgrounds and only the subject lighted up). Great if you want to control DOF and need a realtively fast shutter speed - again at the expense of looseing ambient light.

In M mode, you can control the aperture, just like you can in Av mode.
In M mode, you can control the shutter speed. In Av mode with the custom function mentioned turned on, it will always be 1/250 (or whatever the max sync speed is for the camera).

Seems like M mode gives you more control over your shutter speed and, hence, more flexibility than Av mode does when the flash is in use.