If you sit on max health you lose all of that. As soraka, I avoid that situation as much as possible. I heal, I trade hell I will even heal a minion if that's the only option I have.
Avoiding remaining at max health with Soraka makes sense. Avoiding it at any cost would be artificially engineering a scenario that justifies your build. It's as though we were having an argument about the value of CDR on Annie, and in order to reach your quota of stuns, you started saying that you use your stun whenever it's available even if it means stunning a minion.
You are right that if we look at it in term of potions instead of max heatlh. It become slightly better. But you don't just want to cancel the bad effect of the ruby, you also want to sustain the base hp it gives othewise you'll have to consider that the sightstone is the full 800gp price and you'll need twice as much back to become even in cost.
This is true, which is why I wrote "While still realizing all of the normal benefits of having +150 max health." Return to the scenario where you and I are sitting next to each other and each playing Soraka, we've each used W twice, and I have a Ruby Crystal that you don't. Thresh lands a Death Sentence just as the enemy Vi comes in to gank (in both games). You currently have 640 health, I currently have 760. The extra hp that may allow me to escape while you die, granting the enemy 300g + 150g for the assists. That's factored into the value of a Ruby Crystal. I have more max health. That helps me not die.
Now, in that scenario, I unfairly had a Ruby Crystal while you had nothing. You might have bought an Amp Tome or a Cloth Armor or something else that's helping you. But that scenario is meant to demonstrate that the Ruby Crystal is not a straight-up drawback. It has worth on every champion who doesn't want to die, and on Soraka it has a minor drawback that offsets a tiny portion of that worth.
So to sustain the base ruby, you need to buy 2 extra potions per back.
I'm not sure where 2 extra potions per back came from...
This mean that you lose 1 ward per back. In the best scenario, you'll still need 3 backs instead of 2 to pay for the sightstone. (assuming you use 3 wards per back and that you can place them at optimal position without any opportunity cost which is never really the case).
You're starting to lose me here. If we accept that you offset the benefit of 1 free ward every time you recall (because you're buying potions so that the +150 health is completely not a drawback at all, allowing you to claim "+150 max health is 400g worth of stats" guilt-free), the Sightstone still pays for itself after 3 recalls. A ward-generating machine that becomes efficient so quickly is well worth the purchase.
I never heard of anyone arguing that the avarice blade is a mandatory item for every supports. Even if it pay 1 ward every 2 minutes.
Avarice Blade isn't free though. It costs 800g, while providing stats that have no practical worth to a champion that doesn't help the team by dealing damage through autoattacks. 3g/10 seconds = 1 free ward every 4 minutes (not 2), so it takes the length of an entire game for that item to pay for 11 wards and justify its purchase.
And here's where I get to be just a teensy bit mean. I checked out your history because I was wondering if you were level 30 yet. I see that you are, and I see that you just play support Soraka in Team Builder. I also see that you buy no more than ONE WARD per game, and frequently buy NO WARDS AT ALL. If you were serious about saying "It's better to buy wards instead of a Sightstone because the extra health hurts you," I would expect to see you actually buying wards. To play that many support games and use only your trinket for vision suggests that you have no idea just how important wards are. Even if you completely convinced me that the +150 health Sightstone gives Soraka is worth -200g instead of 400g, I would still buy it because I know it will easily give me 1000g worth of wards before the end of an average game.