The non active part of her w is the strongest bit of her kit. She effectively gets an aegis of the legion, the single most cost effective item in the game, built into her kit. Its an absolutely insane amount of gold advantage for your team mearly by haveing a sona on your team. Sure, in laneing, its not all that great, but once laneing ends that q active is not going to be meaningful, and its only the strength of the passive stats it provides that makes it worth useing unless you have a glut of mana. Her ult is nice, but most of the time your not going to be getting more then two people in it unless you have already won the fight, its her w that carries her, inspite of what people think. Its just not as flashy as the rest of her kit.
As another example of this, tarics ult, his armour aura being a lesser example in its current form. Its an instant baron buff for your team every fight, and when combined with his w gives him enormous passive threat that actualy makes him worth killing in a fight.
Aught to add to this. Its a solo queue vs higher level team issue. In soloqueue, she is a support with very strong poke, decent healing, and an awesome aoe cc ult. For competitive play she is a massive ammount of free stats for the team late game that has the potential to apply a good ammount of pressure to the enemy lane or be safe as required, oh and she has some aoe cc as well. If you wanted to nerf her for solo queue, you would target her early game power, mainly her q, but this one seems to be aimed straight at her late game power.