Just what do these words mean? Entry-level students have quite a bit of trouble distinguishing those terms from Supply and Demand. So we keep repeating: Qd is the amount demanded at the Po price; Qs is the amount supplied.
In our textbook -- and this is something we've added after Heyne departed -- we consider Qs and Qd differently. Yes, what I've said above is true and worth a repeat. But we go further by asking just what do Qs and Qd really stand for?
They represent plans.
Consider Qs to represent the plans of sellers at any given price, and the Supply curve represents a general pattern of changing sellers' plans as the price changes. Consider Qd to be the purchasing plans of buyers at any given price, and the Demand curve represents a general pattern of changing purchase plans at different prices. (Indeed prices themselves represent plans, price changes stem from plan changes.)
Market "equilibrium" is okay to speak about at this level, but that focuses our attention on the end of plan changes, where they are all settled. Better that we also call it market clearing. Why better? When the market clears, the plans of sellers and buyers are fully coordinated, they dovetail, and that's why no further change occurs in the market, thats why the market is now in a state of equilibrium.
And this market-clearing coordination is an unintended consequence of people pursuing plans.
Issues that compare producer and consumer surpluses are fine if one wants to discuss the welfare characteristics of market equilibrium. We can try to compare marginal rates of substitution in equilibrium, but we often lose site of the more important coordination problem, which is what any economy is really about -- including, of course, a comprehensively planned economy. I lament our general judgment of what is important and what isn't.
By the time students move into upper-division courses, as well as grad courses, they've lost the idea that some institutions allow for greater plan coordination, and some don't. (And that assumes students were clearly exposed to the idea.) Institutions influence choices, plans of actions, and they causally influence the tendencies and patterns of changing plans. But we spend more time attempting to discuss the final equilibrium welfare characteristics of differing institutions against an empirically impossible benchmark. The benchmark serves as a normative standard to judge the efficiency properties of alternative economic systems.
Maybe we spend so much of our efforts on welfare properties of the market because the profession takes for granted the market's strong tendencies toward full plan coordination. Maybe, but I don't think so. Consider the Post Keynesians, for example.
This is the comparative systems economist in me (and Pete) speaking. I wonder how many intro courses study the issue of plan coordination. Take a good look for a discussion in your textbook.