Keeping opponents in when they are drawing thin - how bad?

[Ulysses]

$15-30. Decent game. Two complete idiots, two loose passives, four pretty decent players, and one LAG.

UTG limps. UTG is an expert player who rarely limps, especially not UTG. Very hard to say what this might be.

UTG+1 finds QhTd and decides to limp. UTG+1 is short-stacked w/ 3BB left after this call.

Moron in LP calls. Moron on the button calls. OK player in SB calls. BB, a solid, but somewhat overaggressive player calls.

Flop 9d Th Tc

SB checks. BB bets. UTG raises. UTG+1 cold-calls, perhaps expecting UTG or UTG+1 to reraise w/ a worse Ten. LP cold-calls. Button and SB fold. BB calls.

Turn (9d Th Tc) 6d

BB checks. UTG bets. UTG+1 raises two bets all-in. LP ponders a while and says "Dammit, I have to call - it's a jackpot hand" and cold-calls. BB now ponders a bit and mumbles to himself "That may have been a really bad turn card" but goes ahead and 3-bets anyway. UTG considers very briefly and mucks. UTG+1 is all-in. LP calls, clearly not happy about it.

River (9d Th Tc 6d) 6c

BB bets. LP mucks his 7d8d face-up.

BB turns over 66 for the flopped underpair, turned full house, and rivered quads.

UTG+1 shows his QdTh and the player next to him (they are very good long-time friends) promptly chastises him for not 3-betting the flop.

How bad was UTG+1's flop smooth-call? Open-ended straight draws are probably going to call anything on the flop. And one straight card makes his full house. The guy he could have knocked out was drawing to two outs. Given the size of the pot at the time (9SB) and his holding, does he want to kick BB out?