Lupus:
One more reason to use the c64 palette is because it's just simply brilliant! I think it's the best fixed palette in existence. Compare it to cga/ega, vic-20 or speccy palettes [add more platforms that I forgot here] and you see what I mean
You can do great looking stuff with the palette also without restrictions, just see this Indigo's ork coloured with it:
http://ptoing.net/c64orks2.gifAlso, making art with c64 restrictions adds the extra twist that makes it interesting to create. Why people love to solve sudokus?
I think Helm and Ptoing could add tons of more aesthetic etc. reasons to use c64 palette/graphic modes, so I'll leave it to them.