Flashing brake lights are irresponsible, selfish and dangerous.
Have you ever been in a traffic jam, crawling along, and the car 2 lanes over and 5 cars up has one?
It's very distracting and almost caused me to rear end the car in front of me more than once.
Flashing lights are for emergency vehicles only and if someone claims they're legal in their state, city or county, I'll show you the law that says they're not.
Just because a dealer offers them as an option doesn't make them legal. Being illegal doesn't mean that you'll get pulled over or ticketed for it. I think it's in a gray area right now, but if enough people had them, there'd be an outright ban.
Can you imagine if even 1/3 of the cars on the road had these?