So if I cast a fireball but don't roll well enough to hit with it, it somehow doesn't inflict stress on me? And of course there are no meaningful reactions to an attempted fireball, like dodging, that would follow from an attempt. Which is free, because it didn't accomplish anything. Riiight.
You misunderstand me. Say we are in a chase on foot. My action would be to roll athletics to get away from you. But because I'm a wizard and it looks good, I say that while I run, I throw a couple of fireballs behind me, without looking. I don't roll on those, they are not meant to accomplish anything, they are simply flavor for the chase. I could do the same with a gun, but I'm a wizard, so why not make it count?
The point is, that the fireball I throw in that case is not an action, it was never meant to accomplish anything, other than enrich the story. I don't get a benefit from it, my opponent doesn't get a penalty from it, it does nothing but look good. And that's why it doesn't need any stress.
If I am attempting to do something, that's my action. I have the chance of achieving something, so I have to pay for it. If my opponent rolls better, that's bad luck, but I don't get to say "nothing happened, so I won't pay". You pay for the chance of something happening, not for the actual outcome.
It's similar to when Harry does his "ventas servitas" to retrieve his staff. Most of the time it's just flavor. He could have just as well walked over and gotten it, there's no sense in taking stress. But when he does the same spell when talking to the photographer in Storm Front, he uses the same spell to intimidate the photographer with his power. In this case, the spell is supposed to actually accomplish something in the game, it's a maneuver to place something like "look at my fearsome powers" on the scene, that Harry can tag on an intimidation attack. And in that case, it will have cost him a point of stress.
Don't forget all of the relatively long-term spells Harry casts, which involve generating even more stress when they're extended.
Do you have an example of this?