For the record on Rapp, when the Rams traded down, I believe their target at safety was actually Narsir Adderley. Of course, Adderley was the selection immediately before Taylor Rapp, going to the LA Chargers, which might have come as a surprise to the Rams brass, as the Chargers had just drafted safety Derwin James the year prior in the 1st round, 17th overall, and James had an outstanding rookie campaign.
Pure speculation on my part, of course. And even if Adderley was the Rams target when they traded down, I am sure they accounted for the possibility that he might not be available, and as the Rapp pick was in quick if memory serves, I am sure they were content to come away with him as well. Kind of a 1A/1B situation. Which might not be surprising, as Snead has spoken at length that they rank players in "tiers".
I have no idea how Adderley has developed since being drafted. I suspect he is better in coverage than Rapp, but perhaps he is a bust. who knows.