I'm a United Silver and have had a number of flights recently where all E- seats were reserved and many E+ seats were open. On about half of the reservations, United wouldn't let me select an E+ seat during the online reservation process but involuntarily assigned me one when I ticketed. I couldn't switch it without paying but I was given an isle E+ seat (even though my profile preference is window). In the other half of the cases, I was not assigned any seat.

I spoke with a (PMUA) Premier Desk agent who said its a glitch with the website and that when E- seats are completely reserved, Silvers should be assigned an E+ seat every time (not select one but be assigned one). She took care of this for me well in advance of 24 hours.

No idea if her explanation is accurate but it does appear that when E- is completely reserved, Silvers may be able to call in and get an E+ seat.