A drunk text protection app has to assume that anything and everything you try to text, or any call you try to make while drunk is a bad idea. That might not be the case. For instance, you could be ordering a taxi to take you home, or calling for help after getting lost or stranded. The app won't be able to tell the difference and will block everything. This means either that you will have problems from false positives or you will need some kind of "No, I'm totally fine, I swear" override mechanism which destroys the effectiveness of the app.
Mokalus of Borg
PS - I've never had need of a drunk text protection app.
PPS - Is there a stone-cold-sober bad-judgement protection equivalent?