No it won't. Flex still requires people to commit to a specific schedule and commit to being there for a set amount of time. LFR will still be the go to for people who simply do not want to commit to those things.
On the topic of griefing, there have been people right here on MMO-C that have said they kicked people out of LFR for having store bought helms. I have no doubt that these kind of, excuse the expression, assholes, will try to grief LFR. But I suspect that the actual numbers of people like this are small and that those that actually do this will get bored when they figure out that they didn't think their cunning plan through.