I understand, but those MMs usually represent "someone" (an actual buyer or seller).  Every time I place a peculiar "in the money" order to buy or sell, the bid or ask almost immediately moves to my price until I'm filled, or someone else offers a better trade. 

While MMs exercise a great deal of control, I honestly don't believe that they actually accumulate excessive inventory.  For instance, one of my brokers is ETrade, with their own MM.  When I place an "in the money" order, I typically get a 200 share fill immediately, then have to wait for the rest.  Obviously, their "inventory" consists of a whopping 200 shares.

MMs play the spread, that they control to a large degree.  But they're pretty responsive to orders placed.  In other words, the traders/market ultimately have greater control.  If not, the spread between bid and ask would be much greater so the MMs could make more.  And remember that there isn't just one or two MMs in play, but usually at least a half dozen active.

JMHO,

SGE