I think you're all paniking over whether or not it's broken, regardless of whether or not it is a luck based argument.
1/7 is not significant enough to warrant consisent, easy wins. It's overall more detrimental to the team to have a quirky handicap like this.
And it's not 1/7. Because 2/7 of that 1/7 will have defense drops, and the others will have speed, accuracy or attack drops. Good luck stalling against the offensive metagame with 5/49 odds of getting off a boost that only makes you 33% likely to miss a move. No, I'm fine with running a team that works without luck, and gives good consisent and satisfying wins.
And that's even forgetting the counters, such as perfect accuracy and haze, clear smog etc. Inconsistent isn't obviously broken by any means.
Furthermore, consider the pokemon that learn it. None are at all defensive and they are all relatively "slow". Perhaps there is a certain argument of UU, but I see no problem in OU.