It's clutter, white noise, etc. It looks good on overall healing in logs because 5 million sounds like a big number. It's never saved anyone from dying, nor prevented a healer from needing to heal a person taking damage. Players take it because it's passive and don't want to use another keybind.
Doesn't help the raid either most of the time heals yourself, rather than the guy that needs it.
*Cenarion Ward and Renewal for DPS specs specifically are, IMO, better ways to handle the burst damage of boss encounters (weaving it in between or with other healing CDs like NS or healthstone depending on the situation). Ysera's isn't potent enough at times to help as much as renewal and ward can with raid damage.