[bellatrix]

Extrapolate this situation to the river. Remember that situation, where there's some guy that has been betting all along and you only have a marginal hand like 2nd/3rd pair and there is a player to act behind you. If the pot is large enough and there is an according chance that the bettor is bluffing, then you raise. There's an excellent example in Miller's book about the EV.

Now extrapolate that situation from the river back to the turn, simple as that, really. Your equity still is dire against that one opponent, but in relation to the pot that extra bet you put in is still within positive EV if you drive out another opponent. Plus, hey, there's still a card to come...
As you correctly pointed out, the pot size is key and all that makes us barely +EV.

Sigh, I have been working for 30 hours straight, so my thoughts might not come out very clearly.