I believe the reason that the USTA does not want the lines to represent strength is that they dont want to publish more accurate ratings and furthermore they do not want hear complaints about stacking.
People complain that defaulting line 3 first indicates it should be the weakest or by forcing + players to play line 1 indicates line strength ... but it does not. These are arbitrary decisions so that full strength teams can plan accordingly.
The USTA could have just as easily decided that line 1 forfeits first or that + players must play on Line 2. In this way, everyone would know where the special courts were ... ie the courts you likely would win by default or likely lose because you dont have a + player.
"You should be playing linebacker, not singles."