RWC acts as the glue to SMP that tells SMP where to place clouds in a scene based on real world METAR data. Let's use the following example to explain it:
We are going to assume RWC is NOT installed in the following example.
1. San Francisco, CA is reporting clear skies
2. Oakland, CA across the bay is reporting overcast
3. San Jose, CA on the south side of the bay is reporting scattered clouds
Let's say we fly from SFO-OAK-SJC for purposes of demonstration.
As we taxi and take off from SFO, there are no clouds reported in the METAR information, so SkyMaxx will draw no clouds anywhere as far as the eye can see (and OAK/SJC are within eye range). We take off from SFO and head towards OAK. X-Plane now tells SMP we are closest to OAK, so we get the METAR data from OAK showing overcast. Now the ENTIRE region as far as the eye can see is overcast...including SFO where we just took off! It's an abrupt change. We continue flying to SJC, and as we get closer, again X-Plane tells SMP to look at SJC as the nearest station for weather. All of a sudden we get an abrupt change to scattered clouds, including looking back at the OAK and SFO areas where it was overcast and clear respectively. In other words, whatever METAR station is closest will dictate what SMP draws for the ENTIRE region!
Now, if we had RWC installed, it would analyze all the METAR stations and tell SMP to draw the clouds differently in those three airport areas. Clear over SFO, overcast at OAK, and scattered clouds over SJC. You could get up in the air, look around and see each area reflecting that information accurately. RWC is key to interpreting METARs and telling SkyMaxx where to place different weather fronts rather than drawing the same type for the entire scene. There are no abrupt weather changes as you move from one METAR station to the next.
The answer is a mix of 'Yes and no'. RWC is capable of downloading real weather METAR data from the same NOAA source as X-Plane gets its default weather. ASXP claims to have better weather station reporting, so the theory is that ASXP is providing more accurate weather information to X-Plane (and thereby RWC/SMP). Whether it actually does that is up for debate. I do know it has a better wind/turbulence model than default X-Plane can do. You will want to run RWC in the External Injector mode for use with ASXP.
Our recommendation is FSGRW+RWC+SMP. This is a killer combo that will provide the most accurate and best weather across any other option. The reason for this is because FSGRW has been created to add in extra cloud types that SMP can handle and display, meaning you will get a much more accurate picture than you will with any other weather injector.
Ultimately it's going to come down to your own personal preference, but I hope this sheds a bit more light on the situation for you!