Soraka's healing doesn't keep up with eg. a single full power cord poke combo until it reaches level 3. Good soraka's can disrupt that but then you've also got the adc trying to poke you on top of that. And the problem is you continued to be outgunned with Soraka even post 6 because you pay a heavy price in lane power for the global ult. They even nerfed the cooldown to 160s from 120s while eg. Sona gets her big gun every 150s and it's much bigger in lane. You can help other lanes with it, but I would strongly argue that outright having control of bot lane is a much bigger help to the team as Soraka currently stands.
I'm not saying they are so totally trash you can never win with them but the reward vs risk with them is such that they aren't reliable any more and other supports can give much better rewards.
Again I already mentioned what soraka can do to a adc or support trying to poke her. On mumble or vent coordination can make up for a lot I speak from my own personal experience of using her but also having good duos. GG doing your combo vs smart people after a few times they will make you pay for it. Oh you're coming in silenced and you will be losing more on trades. Sona dies like anyone when focused and if she can't cast or trade without winning she is dead like any other opponent who relies on their stats vs smarts.
Soraka doesn't need to make up for what you mentioned. Ever try soraka with defensive focus sona's harass is crap with mr 60+ during the early game.
There is no broke support on bottom most can be countered by positioning, good trades, and ganks when they go over aggressive too quickly or as certain gaffers have trolled with me at times wards/teleport.
I'm the last person to get in to a risk/reward argument. I'm one of the few gaffers that for themselves beats odds a good portion of the time. If I took what you said seriously I wouldn't use the characters I do at the times do and pull off the trolls I do. For winning a consistent majority of 3k worth of games think I get to speak for potential vs probability.
edit - probably won't have a new machine as expected soon parts were being shipped out of NJ at the time of hurricane sandy.