A filler is that skill or spell you do when you have nothing better to do.

In the Summoner's case:
1) Your DoTs must be applied at all times to all intended targets.
2) Use Ruin when you don't need to do step 1.

Ruin is the SMN's filler spell, much like WoW's Shaman's Lightning Bolt.
Now the subject here is what is most SMN's is their Lightning Chain, which is the WoW's Shaman's AoE Filler.

For the longest time players have already determined what is now Tri-Bind to be ineffective as an AoE Filler.

What bugs me is why nearly all of the never use Blizzard 2? Despite having to be close, it's cheaper, faster, and stronger than Tri-Bind, and slightly weaker than Ruin. It's also a good follow up for Miasma2.
And I'm talking about when it is obviously safe to be close in a mob of over 4 enemies.

I find it very annoying to constantly see only myself take advantage of Bliz2 in huge mob battles.

So what's up with this?