Using this scenario about "telling the kids they are not good enough", what about using the Flex/DP? Aren't you telling the Flex that she isn't good enough to bat for you, or The DP that you are a libility in the field?
I don't use Flex/DP partly for the reasons you're suggesting, but I also don't think the use of the flex/DP is the same message as asking kids to make outs. The Flex/DP is a case of telling someone that another player is better at a particular job. The lineup and batting order always send those kinds of messages. Players are used to that. To me, that's different than saying that we can't win this game if we're depending on you.