I think the emotional size of a +3 bonus may vary with personal experience, so I'll concede that my own d20 history may not be representative of how it "feels." But I think I can still use some specific examples to make my overall point a little clearer.
Let's say we take that 1st level character, who is facing a difficulty 7 task (pretty high for level 1). Now this hero is reasonably well-suited to the task, and has both a relevant skill and asset. It is now reduced to a difficulty 5, which means he needs a 15 on the d20. Figuring that this is still pretty high, he (understandably) spends a point of effort, so that now he needs a 12. Now, the straight up math seems to suggest that this increases the likelihood of success by 15%, as you say, but let's take a look at the three possible outcome ranges:
1-11: Failure. But more importantly, this hero was not going to succeed with or without the effort. The expended effort is wasted.
12-14: Success. And the hero feels righteously justified in spending the effort.
15-20: Success. However, the hero would have succeeded without spending the effort. Effort is wasted.
Now, maybe you can say that the 15-20 range still feels worth the effort if it was something important. You wanted to make sure. You got a good roll this time, but it COULD have been slightly worse. Maybe some people can think this way, but too many of those and I'm going to get irritated.
Second example, same character. This time the character is facing an easier task that (with bonuses) is reduced to a difficulty 2. Needing a 6, the hero sensibly decides this is not the time for effort. Let's look at the spread here:
1-2: Failure. Wow, bad dice luck. Well, that'll happen. Nothing you can do about that.
3-5: Failure. Uh, crap. Why didn't I spend an effort just to make sure?
6-20: Success. See? No effort needed.
So here we are on the other end of things, and once again what I'm seeing is a 15% chance that I actually care about effort, and an 85% chance that I don't.
But then there's the question of the higher-level character. The thing is, the frustration gets WORSE as you have the option to spend more points. Now, I am less familiar with the higher level stuff, but don't you have to decide how many points you're spending? If you just get your max every time, that seems pretty swingy. Assuming you do get to decide how much to expend on each roll, I have another example:
Let's say you're a 4th-tier character, with the option to go up to effort 4. You're dealing with something that ends up at a difficulty 6 without the effort. So...how much should you spend? If you spend 3, then you have the following outcome ranges:
1-5: Failure. Wasn't going to happen no matter what, but those are pretty low rolls, so I don't feel bad about it. Still, I wish I hadn't wasted that effort.
6-8: Failure. Argh! Why didn't I spend more effort? But how could I know it would end up here?
9-11: Success. Yay! I guessed right!
12-17: Success. But...I didn't need to spend that much.
18-20: Success. Well that was a waste of effort.
And if you spend more or less, you are simply moving these ranges around, potentially expanding and contracting most of them, BUT NOT the "Yay! I guessed right!" result range. That will ALWAYS stay 15% no matter what level you are, no matter what the difficulty is, and no matter how much effort you expend.
So, I hope this at least explains my feelings. If you have a different emotional reaction to these various ranges, then you probably won't mind the mechanics as much. I am certainly willing to admit that this is a YMMV situation. I just wanted to articulate more clearly what it was that bugged me about it.