I like your analysis. I will agree that proactive shielding is far more difficult to pull off than reactive healing.
However, healing is affected by Grievous Wounds, which almost halves its usefulness,whereas shielding is unaffected by it.
Janna's shield lasts a very long time, forcing you to trade while it's up, or disengage and wait for it to fall off. It has a downtime of only 5 seconds, not including any CD reduction. Janna's shield also happens to be instant cast, so whether or not I can surprise her with damage before she puts her shield up is based entirely on her reaction time.
Soraka's kit also forces her to interact with enemy laners, which limits how much she can do depending on how good her opponents are at dodging. Janna's shielding is only limited by her mana. Personally, I never see Janna's starving for mana nowadays, but I could be wrong on that point.
Both are frustrating, but theyr'e different kinds of frustrating. Soraka erases trades after they happen. But Janna shielding her ADC means I will lose any trades for the next 5 seconds, no matter which ADC or support i'm playing, unless their ADC makes a very bad play.
I really wish it had either a longer CD, shorter up time, or a cast time. Maybe it wouldn't be as bad if it was affected by grievous wounds, but I don't think it's really that bad outside of laning phase because team damage can usually burn through it quicker than she can put it out.