David
The good thing about using real weather is that you can use real predictions. If you were to generate some kind of random code that decided the weather, that would mean you'd also have to develop a prediction tool too. So basically, you'd have to first program weather patterns for each track, then you'd have to code a 'rough' system for predicting it which might be only 95% accurate or something.
Of course, there's nothin wrong with doing that.... it's just more stuff to do.
If the game were to use its own weather system, I imagine the prediction tool would be simple and not a lot of work. The weather system might have its variables and whatnot, then the approaching weather would always be delayed by 15 minutes (or whatever time), giving room to a prediction tool that can have its accuracy controlled. By being able to control the accuracy, the developers can choose how the prediction tool is available: either to everybody with a set accuracy or implement it as the weather station facility that can become more accurate when upgraded.
One tough part about creating the weather system could be on making one separate weather system for each track. If such a thing were to happen, I'd preferably like to see even more variable weather for each league's race, regardless of track. (e.g. two separate leagues racing at the same time at the same track, but both are experiencing different weather conditions)