It's not situational, it's useless. If you have her on sic she'll blow it on uninterruptible casts, and if you have it her on obey, who knows if she'll even get it off? Recognize that a spell is being cast, that the spell can be interrupted, command Selene, wait for Selene to recognize you commanded her, wait for her GCD to reset, apply the silence, then hope to god that the server even registers the silence even applying before the server says the cast is finished. Unless we see a massive improvement in pet responsiveness or we start seeing 5+ second cast times, no group will ever even consider Silent Dusk.