Since I personally don;t do a lot of Domestic I can only give you my experiences in OP UP's on International flights were it jas happened to me thus far 6 times receiving them but I find unless a flight is oversold these days you are most probably but NOT ALWAYS going to get an OP UP if there are empty seats in the back.
These E-Credits are actually helping AC in better managing their Yield Management practices especially when people request their E-Upgrades well in advance.