I don't think its correct to frame the discussion over the Cousin's pick as BPA vs drafting for need for 2 main reasons:
(1) Cousin, a developmental and hopefully an effective back-up QB, was a need pick
(2) When most teams draft a player that player is the highest rated or BPA on their draft board otherwise they wouldn't select them; the discussion then isn't about BPA vs need, its about whether their evaluation of the player was correct
What is Kirk Cousins worth?
Cousin's is an unproven developmental back-up QB. I think the FO office would be hard pressed to get a 5th round pick back.
But, if Cousin's progresses and plays well in real games he could raise his value and have greater return then his selection, but right now its just a hope.