Latence was doing this build, but he's waiting until he hits challenger to write a post (wish him luck!) and it's the first dramatically new and maybe viable change in a while so I wanted to write something while it's fresh.

First, ingenious/ghost poro are really good and affect a bunch of interesting items - see http://www.legendsofleague.gg/league/post/116/Ingenious%2520Hunter%2520%252B%2520Crown%252FSeraphs/ for champs like seraphine or lux.

Second, soraka usually doesn't get great value out of moonstone/shurelya's because she doesn't spam spells as constantly (think vs someone like sona or milio).

There's more to why the items you get instead are good, but I'll leave it here for now and wait on latence hitting challenger for his thoughts.

edit:

Well latence got to like 800 lp then something bad happened and with the item rework now, which changes all the enchanter support items and adds echoes of helia, I wanted to get this down now.

Soraka doesn't apply heals/shields very often, her w has a health cost so she can't spam it like other enchanters do. So since all the enchanter item effects come from healing/shielding people, soraka gets less out of them. If you tab and check moonstone/shurelyas you'll be underwhelmed compared to sona, lulu, karma, nami, annie, whatever. Even staff of flowing water which is a good item idk how often you can apply.

The way soraka plays teamfights mid/late game is to interact with enemies as little as possible, like press w on allies and just run away from enemies. Maybe you e or q them but even that is more when an enemy goes in. The main point is it's never good to lose your own hp bar and it's not like you do anything offensively. This works really well with crown/banshee's, because 1) their 40-27s (ingenious hunter) cd resets every time you take damage, so having the shields up is much more realistic on a champ like soraka who avoids interacting with enemy champs and 2) a lot of enemy champs will really tunnel on the soraka every teamfight, like prioritizing you over literally every other target, and you lack self peel, so the shields tend to be valuable in teamfights.

In terms of your actual healing numbers, I'm not even sure it's less healing especially given that the reduced damage taken lets you cast W you otherwise wouldn't be able to cast. Ofc you can say "just position better to not take damage in teamfights" and yeah that's the best point but a lot of the time people do just dive soraka.

Also, a maybe underrated point is a lot of games are decided early - I'm still working on seeing exactly how many but idk, a lot - so what really matters is your early items. And for soraka I think the ideal early build is blue item, boots of lucidity, refillable potion, fiendish codex. Alternatively amp tomes over fiendish codex. Dark seal if you can stack it.

These items have no mana, which is good and bad. Again soraka is limited by the W health cost (which is why refillable is great, and why crown/banshees give you surviveability without increasing W cost), so usually you don't run out of mana before health. But sometimes if there are multiple extended fights and you never recall and throw out a lot of Q/E (which cost a lot of mana but 0 hp) you can run out of mana and then be totally useless. So idk you defnitely don't want as much mana as other enchanters, but sometimes none can bite you or maybe you just have to be more careful with it.

The other thing that's good and bad is refillable + dark seal are both super cheap and super gold efficient, so you can get them early and be strong, but at some point you can run out of slots. In particular when you're on support item/boots/refillable/dark seal/banshees or crown, that's 5 items, so the next of banshee's or crown you only get one item slot to get components for. So here you'll have a fiendish codex or lost chapter in the last slot and then recall with like 1000 gold and be unable to spend any of it, which sucks and is the biggest downside of these items (the refillable+dark seal but also the banshee's/crown build paths). Still, the early game of just getting boots, refillable, and ability power is super good and worth it.

As for which of banshee's/crown to get first, banshee's is a bit better vs heavy magic damage and ideal if there's a cc like a blitz hook you don't want to get randomly caught by. Idk though which is better first.