What type of Mac are you using? On 2009 and earlier MBPs you had to control it from the control panel - then log out. Luckily gfxcardstatus can swap without having to log out.
On the newer 2010 i5 and i7 MBPs it autoswitches depending on need - or as schweb said, that is the theory. Everything I read says that it tends to stay on the discrete card no matter what.
Anyway, I agree with schweb - the best way to control the card is with gfxcardstatus as you know what card is being used. It runs in your menubar and has a letter corresponding to the graphics card being used. (n for discrete Nvidia i for internal intel) and you don't have to log out to swap it.
Power Savings is the main reason. If you aren't doing anything heavy it really doesn't help to run the discrete card. When I use the internal card, turn down the brightness of the screen, turn off keyboard backlight and bluetooth - I get the ~7 hours of run time reported in my battery meter. With the discrete card the most I've ever got it to report is 4 hours of run time.
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