While I agree with you in general about the quality of play when there is nothing really on the line, I have found some modicum of value in play money SnG's.
1. I use it to practice discipline and spew avoidance. It is very easy to fall into the trap of shoving preflop with trash because everyone else is; however, once you realize you are truly better than those players, you can maneuver your chips to greater advantage and dominate.
2. Even though many players at this level will play any two cards, I have used it to begin thinking about poker strategically. For example, it may be worth adding some trash to your range knowing others will pay off with their entire stack when you hit the flop huge. It isn't unusual for these players to pay 100BB or more with TPTK when you have a monster. When you don't hit, you can fold cheaply.
3. There is value in just playing
hands to develop experience. The ability to read a board quickly and know your 2 pair isn't really a good hand comes through repetition.
On a trip to a brick and mortar room after spending more time at play money online, I noticed a significant difference in my level of play, and I attribute much of this to that practice.