I'm not usually one to call a champion useless. Underpowered sure, but still playable.
An exception can be made for Soraka. Our poor star child.
For those of you who don't know, Soraka was a godess durring the healing/poke meta game. However, when that meta game ended, so did Soraka's strength. Since the introduciton of Sona there seems to be no point in playing Soraka, since Sona does just about everything Soraka does, only better. Soraka has her mana restoration and silence yes, but Sona has the damage, movement speed, and the aoe stun. Sona's just a more viable support.
Consider this. The last game I played in SR had a Soraka on our team. She was by no means a bad player, but I could tell that the she wasn't cutting it. Even with the healing done, she was nothing but a target for most of the match, and her deaths went mostly to their Pantheon, who spent the game tearing us apart. We eventually won, mostly due to our amazing ramus (damn man you rocked), but I still felt that we could have done better with a support other than Soraka.
Now don't get me wrong. I know that there are players out there that are good with her, and that she does have the pontential to be a good support. And I know I can't comment too much on her since she is one of the few champions that I have not actually played. What I notice the most however, is that every other support champion seems to outshine her, with Sona pretty much replacing her. It's kind of sad to see a champion literally replaced by another.