A faster CF card is most useful when you shoot bursts in data-heavy modes like RAW+JPEG. The main benefit is clearing the camera’s buffer faster, so the 7D is ready to keep shooting sooner. It can also shorten transfer time when copying images to a computer.

If you don’t shoot video and only use short bursts, a slower card can still work. But the 7D is a fast camera, and RAW files are large, so in continuous shooting you may notice the advantage of a faster card.

In practice, card speed only helps up to the limits of your camera’s write speed and your card reader/computer transfer speed. So the fastest card available won’t always give a full real-world benefit.

Bottom line: for occasional non-video use, a slower card is acceptable; for frequent burst shooting in RAW+JPEG, a faster card is a sensible choice if your budget allows.