Well first, here's how crossfire/SLi works:
The cards process frames one at a time, back and forth. Card #1 renders a frame, and then card #2 will render the next frame.
Lets say you get 60FPS in a game. That means that every second, Card# 1 renders 30, same with card #2.
Now, the only difference between these two card and a single card is the Latency between each frame. With a single card, the latency is uniform, each frame is rendered consistently. In a dual-card setup, one frame might be rendered in 15ms, but the next frame could be rendered 1ms after that, resulting in a 14ms delay before the next frame is rendered. It isn't terribly obvious, but it can sometimes cause the game to hang for a very split second, once a minute or so, and it can be annoying,
You might not notice it, but if you move to a single card setup, it's a moderately sized difference in appearance.