The thing with shiho is not her increased atk, it's her permanently increased attack speed. It puts her on a different healing speed than most healers (her SAW makes her switch between attacking and healing iirc) meaning she is that healer that might just save your unit with a quick heal that a lot of other healers can't. She is not alone in this, but she is an event character which makes her easier to get than, say, Saria.

If you want healing throughput you pick something else, if you want safer healing while you take constant damage, Shiho is great.