Your argument assumes that sales follow from clicks (and can be tracked in the same way), so one need only follow the clicks to a sale. But this doesn't take into account that many sales take place long after a client has browsed online advertising. This is particularly true for big-ticket items.
> That PPS is a more trustworthy metric than PPC.
Yes, true, but only for sales that follow directly from an advertisement, with no intervening time or context changes.
I agree with the basic argument the PPS is more reliable and meaningful, but it would be worthwhile to know how many sales follow directly from an initial advertising exposure, as opposed to a more complex decision-making process.
Consider the diamond campaign waged by De Beers described in another HN thread today. The advertising costs were high, but the goal was to change consumer perceptions over a period of decades (and successfully). This is far removed from the model we're discussing, essentially an impulse purchase -- the De Beers campaign wasn't directly correlated with diamond sales at all.
My point is that not all advertising can be shoehorned into a directly trackable purchase, yet those other kinds of advertising might still be valuable.
