I pretty much agree with you. As someone who has mained Thresh since season 3, I have always favoured getting an extra tank item (and no, I don't count 400 health as being a tank item, it's basically an incomplete Giant's Belt) over Sightstone, and it has generally worked out favourably. My early game build order on Thresh is always
->
->
->
. Those items give me enough sustain and tankiness that I can usually go all-in on the enemy bot lane and win the trades, often picking up a kill or two for my ADC. And if we don't have enough vision and my trinket is on cooldown and the lane is pushed, I'll just hang back a bit behind my ADC and if I see the enemy jungler I'll immediately throw out my lantern and get him to safety, which prevents either of us from dying to ganks 90% of the time (assuming my ADC is smart enough to click on my lantern).
Then my late game build will consist of 4 tank items and often one on-hit item (
or
, depending on how much money I've got), and I'll be pretty much unkillable while being able to kill their ADC by myself in a couple of seconds.
Basically, I feel like the extra combat stats I could get without building Sightstone help win the lane, and go on to win the rest of the game, more often than the extra vision helps control objectives and avoid ganks. And at level 9, upgraded yellow trinket is pretty much a slotless Sightstone for 250g.
That said, on a support with less peel and disengage like Soraka, I would still definitely get a Sightstone because ganks will have a much higher chance of killing either me or my ADC (or both) compared to if I'm playing Thresh, and being able to clear enemy wards with a Sweeper is a big help for taking objectives or ambushing.
TL;DR If I'm playing a support with heavy disengage potential, who can usually save both myself and my ADC if we get ganked, I won't take Sightstone. If I'm playing a support with bad disengage, Sightstone is definitely a worthwhile buy.