At least in the FAA system dispatching a flight is based on forecasts. The reg actually says "weather reports or forecasts, or any combination.." But I've never seen a forecast with RVR predicted. If the flight is only 30 minutes or an hour the current report matters as much as the forecast but as the flight gets longer you're going to base your planning on the forecast. Unless you're going to Hawaii where the current METAR will be about the same all week.