Righty, I actually don't know how Erriond is scoring painting exactly this time round (I do general organisation, tell people where to go, what to do, etc -- Erriond does the more important tournament stuff) but it will go something along the lines of (hypothetically if a win was 3 points):
Undercoated army: 1 point
Based army: 1 point
Painted army (three colours etc): 1 point
Usually if only half your army is at one of these stages we award half points, so half your models based will be 1/2 point.
So if you have a fully based and painted army you will basically get the points for an extra win by default, which of course is very helpful in the rankings and pretty much essential if you want to come first.
However, by no means do you have to have a painted army; one of my friends who is a very good player never comes with a painted army but he just likes the challenge and to get some good games in.