No offense (truly and honestly don't mean offense) but when people say stuff like this it really just tells me how out of touch they are.
Especially for competetive fellowships you dont need "good" letters. You need "off the wall" exaggerating letters. If you can't understand that, youre out of touch to me. You're only as strong as your weakest letter these days. If your letter writers can't play the game, then that is to your detriment.
That being said, if you can't respect that wish then that is absolutely your perogative and I actually would appreciate the honesty. If someone were to tell me something to the sort of "I cant speak to this specific quality that you desire" then that puts the ball in my court in deciding whether I want to proceed, which is actully better in the long run
TLDR: I would cut people some slack for "guiding" their letters. They are in charge of their application.