Correct - though it's still interesting to stockpile mystery shards as much as you can, because at the same time as the double summoning rate events, there are also Champion Chase or Summon Rush events, that give rewards if you reach a certain point total from summons. And if you've been slowly building a stockpile of mystery shards so you have a couple thousand of them, it means you can reach those rewards without having to use other shards.
So basically, use mystery shards for daily quests or if you are running short on champions to use as food, but don't pull everything you have trying to get better champs (there is a very small chance of getting rares from a mystery shard so once every fifty or so shards, you'll get lucky).