Actually we ARE talking about reading all the limit orders on the ES book, thats the "depth" in depth of market.
My next point was how "time of sales" the actual "tape" of transactions is more important than what is displayed as limit orders; and the histogram gives a better read... a "market profile."
My next point was that even the "tape" of the ES, is better than the limit orders, but still does not represent the S&P cash as a whole since there are other associated derivatives SP, SPY and their derivative options contracts.
Right the ES does not trade in a pit, but the SP does; and nothing can stop one from buying SP pit and laying off in the ES, or Buying the ES and Laying off electronic SP or the pit or both.
Take it a step further and "lay off" off exchange .... across to dark pools.
So you dont know the REAL exposure and commitment in the market as a whole...Maybe with the COT report on each Fri. and still its even a week old.
So, the depth of market, DOM is best used as an order entry tool ( since some have sophisticated properties), rather than to gain insight as to purposed intent/exposure and direction in the underlying.
Finally, one of the best "READ" features of some DOMs are the transaction histograms, but again its just representative of the derivative on the DOM, and not the composite of the underlying.