with the amount of injuries to WR and RB so far this year, I would personally only roster a 2nd QB to fill an immediate bye week need if you had a stud QB already. Early in the year with so many committee RB situations I think having a share in some of those could prove to be valuable down the stretch as more and more guys get hurt or drop off. For WR's its less of an option to hoard since most teams are spreading the ball around pretty well these days, but early on in the year I like to swap out that QB spot for an extra running back option and hope for a home run (like what Langford, D-Will, David Johnson etc) were last year.
Rivers is surprisingly injury-proof in numbers, despite having massive injuries in the last few years on that offense hes always in the top 10ish on QB's and you can get the guy so late in drafts. He finds a way to get it done but it sneaks up on everyone.
I have no problem with Cincy D versus the dolphins, I think the Phins offense is in a rough spot and Landry looks to be beat up pretty good. Cincy can grind up some serious clock with Hill / Gio. Dolphins would be one of the teams I would be targeting for streaming opponents against (along with the Rams and Cleveland)