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I'll try my best to clear up this confusion. Let's say your 3D system really does require a refresh rate of 24 fps to sync with shutter glasses (to say I'm skeptical about this would be an understatement). If true, it would need EXACTLY 24 fps. The timing for left/right eye is extremely critical. We're talking hardware precision here, not software.
When my program sets a frame rate, it simply records the time as it exits the loop. The next time through the loop, it checks the elapsed time. If it's less than that required for the desired frame, it waits a moment, then checks the time again. JUST CHECKING THE CLOCK burns up several thousand cycles, and that alone would insure that the timing can never be exact. But things get far worse. At a certain point in the loop, the program reliquishes all control back to Windows (hoping to get it back soon). All the other programs in the multitasking system then have their chance at clock cycles, and may or may not EVER give control back to my program.
So, even if you were allowed to set a frame rate of 24, that's just a high-end request. The frame rates for 5 consecutive frames might be something like 23.2, 21.9, 23.9, 8.1, 23.9.
For this reason 3D cards set their own refresh rate in hardware. There's absolutely no reason the system should care whether the 3D coordinates in the frame buffers have changed in the last 1/24 of a second. There's no traffic cop that says, "Hey, I suspect that these numbers have changed twice since the last time I was here. I'm citing you for excessive speed, and shutting down your operation!"
Also, the reason film doesn't look too bad at 24 fps can be summed up in two words: motion blur. Moving objects are blurred between where they are now and where they were in the last frame. With a 3D program, moving objects get very ugly and jittery below about 30 fps.
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