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 Exsanguinate with other players who are playing blue might be really hard.
Basically, what Quicken 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 Quicken, 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 Quickens 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 Quicken. Tooth and Nail. Rite of Replication, kicked. Instant speed Damnation right after your opponent Genesis Waved a bunch of creatures on the battlefield with Urabrask the Hidden among them. A huge Genesis Wave of your own. Even (relatively) early Time Warp 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.