Here's 2c worth of experience:
I flew happily for years with default weather and tried all sorts of textures for sky and cloud.
Being FS9 I just max everything: visibility, cloud draw etc.
And it was Not Bad.
Then because many FS forums said AS was a must, on a whim I bought AS6.
First flight was Siberia in winter, and it permanently changed my approach to flying.
It just felt more...real.
Not sure exactly how it does it, but the weather engine is pretty complex, and creates incredibly complex visuals.
How do I know? Well that particular flight in Siberia returned 6 FPS
My computer eats FS9, so that was a bit of a surprise (although it hasn't happened since)

AS returns great, and realistic, weather patterns everywhere, in NZ too.
I would not fly without it underpinning all my custom cloud and sky textures.
I imagine FSX would benefit in the same way.