There are two things to consider here. You create colors on a display like this by turn off/down certain sub-pixels. To create a solid black line, you turn off all the sub-pixels in an RGB stripe. In a PenTile display, however, you don't turn whole pixels off because the pixels don't have real physical coordinates.
Combined with the fact that, say, next to each other RGBG pixels still appear white (and not some super green white,) there will be instances where you end up making a black line by turning individual sub-pixels off. Where a hi-red, hi-green, low-blue color meets black, you will end up with something like (X means off/black) this:
XGRX
RGXX
XGRX
RGXX
Which results in a fuzzy edge.
If you then take a 2:1 ratio, it would look something like this:
XXGRRX
RRGXXX
XXGRRX
RRGXXX
Which makes the edge look even worse.
Of course there's a lot of 'magic' that goes into blending pixels together, but you get the basic idea.
RGBW actually gives the power savings. Because the white pixel doesn't affect colors, it effectively increases your brightness by 25% at the same backlight power. Unfortunately it also increases the distance between colored sub-pixels, making blending less effective, and thus harming color reproduction.
RGBG gives more accurate colors though.