And after ten of those times when you were confronted with unintelligible minutia that didn't change anything except minor, minor improvements (oh, wow, twenty pages on how to select the exact pattern of polka dots I can now select for my hair colour change! Oh, and an further 35 pages on how to change individual hairs of my back! Wow, that was productive!), most people would give up.
Good for you. But you are not the universal measure of mankind. Most people that after getting the spell can fly without reading all that, and have been fine, will continue to fly the way they did the first time. If they have an issue they want improved, they might -MIGHT- check, then be confronted with unintelligible minutia, and give up.
Yes, they aren't computers. Computers are significantly more useful, powerful and dangerous. And yet 99% of people with computers don't know how to fully use them. They just use them to read their mail and check websites.
And for what feels like a hundredth time: Elliot had a specific problem of unwanted transformations that would have been helped by him reading the manual. Susan didn't have a problem, wasn't intending to gain spells, and made reasonable if incorrect assumptions about the damn thing. There is nothing universally stupid about not keeping up with the manual if you already have the powers that you want.
Grey Wolf