I don't know a lot about SCART but here's my guess:
SCART (RGB color, PAL regions) provides a level of picture clarity noticeably above the usual "yellow" composite cable. S-Video (YUV color, NTSC regions) is also better than composite, but SCART(RGB) also manages to be clearer than S-Video. SCART(RGB) also allows the 60Hz option for compatible games&TVs, but does not provide progressive scan. And according to some PAL gamers I've asked, progressive scan was disabled for all PAL GameCubes. So SCART(RGB), running at a clear 480i @ 60Hz, is the way to go for a PAL GameCube.