Its not hard to come up with scenarios where you might want to save your spells in EDH to use them at the appropriate moment. After all, finding that just the right moment to cast your immense Card not found with other players who are playing blue might be really hard.

Basically, what Card not found does is give you more windows. Lets say you are player A, and both players B and C are playing blue. Its B’s turn and C is tapped out. Now, normally C would untap and you would be unable to take advantage of C’s situation, but with Card not found, you have an additional opportunity right then and there to do it. Granted, it will expose you for the duration of C’s turn, if you can’t finish him, but the opportunity is still there.

The really good thing is that this opportunity doesn’t really cost you much. Only one mana. Granted, there are plenty of one mana cantrips out there, but this one has the additional benefit of not requiring a target, like so many of them do. I saw one standard deck, which put a full playset of Card not founds into his main deck despite only having sorceries in the sideboard, and there weren’t that many of them either.

You can get up to all sort of shenanigans at end of turn with Card not found. Card not found. Card not found, kicked. Instant speed Card not found right after your opponent Card not foundd a bunch of creatures on the battlefield with Card not found among them. A huge Card not found of your own. Even (relatively) early Card not found will be better if you have your full mana available on the first of your two consecutive turns.

In short: Possibilities: Endless, cost: negligible.

So, use it. I don’t see any reason not to. Maybe if you don’t use any sorceries in a blue deck, you might want to use something else instead, but that’s not too common.