I think you make a very good point about the importance of segmentation. I like to carry that analysis even further, and measure what the expected return is per segment. If I get a 4.2% conversion rate from email and 2.6% from ppc, the question of the average spend from an email customer versus a ppc customer also factors in to the calculations (assuming revenue is your objective).

So many ways to slice and dice…the other big question, of course is identifying how to break down the segments. Demographically, by channel or what have you.