CTCSS, PL, whatever you want to call it, I'll use 'tone' to refer to it here.
Does the use of a tone ruin GMRS/FRS? Given that the users are not familiar with communications equipment, and that most users don't know anything beyond selecting a channel and pushing the PTT button, we cannot expect them to understand the use of tones and what they do/do not do.
Because of bubble pack radios coming pre-programmed with tones, people will generally only hear those who have the same radio. While the purpose of FRS/GMRS isn't to find people to chat with, it also loses the possible utility in emergency situations. For instance, there may be 5 groups of hikers, each with their own separate set of radios in a National Forest hiking when someone is hurt. Even going through every channel calling for help, the person would not be able to reach another party that may even be close by.
It also seems that it is unlikely that people will check the channel for activity before use, causing interference with other parties.
While I can understand the need to use tones in urban areas because the amount of traffic may get annoying, the reality is that the tones just create more of an inability to coordinate and share the frequency.
Why wouldn't we really want every radio to come out default with carrier squelch?