You can ignore ordering but only with additive blending (common in many particle systems).
With "normal" multiplicative blending, ordering is important. The color buffer accumulates colors. New translucency effects are blended using a source alpha and the inverse of that source alpha. That is, given current color
A and source color
B with source alpha
C, the final color
D is found using
If you're looking at a single component, say the color buffer has value 100, trying to blend in value 50 at 10% then value 100 at 30%. You get
100*.9 + 50*.1 = 95, then you get
95*.7+100*.3 = 96.5.
Swap the order of the blends. Now you get
100*.7 + 100*.3 = 100, then
100*.9 + 50*.1 = 95.
Two different final color values based on the order they're applied in. Hence, order is important if you are aiming for the "standard" blending equation which "looks right."