If the cards display frames as they're available, they end up with uneven display times, which is a quite jarring visual effect. If they VSync they end up with the standard VSync issues where occasional slow frames are difficult to handle. Both are undesirable traits for VR. An output buffer naturally adds latency, which is also undesirable.

The card-race issue is called "micro stuttering" and you can read some outdated information about it here: http://en.wikipedia.org/wiki/Micro_stuttering .

AMD cards in Crossfire used to be notorious for this effect, but they've since added an adaptive frame-rate limiter (forcing fast frames to delay, like VSync but not bound to the display's refresh) which helps.