Even without cards and talent choice, Seris can heal multiple allies at the same time (at a lower rate than the primary heal target of course). Also you can target an ally with Restore Soul and another ally behind a wall can be "chain" healed. This comes in handy when the ally needing heals isn't visible or in your range but is in range of a "chain" heal.
To heal multiples with IO, you have to use the Life Link talent combined with Luna and you still don't have all the benefits Seris has.
I'm not bashing IO at all. The weapon is much superior to the slow traveling orbs. There has to be trade offs.