i would make two points
1. iron discipline. If he raises substantially i fold, no matter how much i want to showdown. If opp value bets it i guess i show top two pair, thats how id play here if i had the set.
2. If i dont block bet the river what option do i have? if opp bets anything but a large bet or pushes im basically limiting myself to a call. I block bet the river to encourage opp to 'show' me what he has by coming back at me. It may be more expensive but in this scenario im not too sure how good my top two pair is. I could be marginally ahead (as it turned out) or way behind to the set. I want to dictate the terms here so i gain enough info as to whether i call a reraise suspecting im ahead in a marginal situation or as to whether i dump a good hand. checking here tells me absolutly nothing about opps strength imo.