Hrm... No, the idea flies in cannon. Madoka is on the receiving end of all kinds of suggestions and Kyubei implies that whatever she wishes for, no matter how ridiculous her friends suggestions are, he'll grant it. This lends food to the idea that Kyubei is really a construct with a simple personality. It explains so much:
Why his facial expression never changes
Why he seems to be cute without actually acting the part
Why he just doesn't get people
Why he seems to have ulterior motives
Why he frankly just doesn't give a damn
And last, but not least... Call me crazy, but while he seems perfectly acceptable, if a bit odd at first, prolonged exposure puts him firmly in the Uncanny Valley.
EDIT: in an odd way, Heisenburg's Uncertainty Principal applies here -
it's hard for Kyubei to tell if any wish she has was not influenced by
anything but her own desire. Also consider, he's got a 'business' to
run, so he can't afford to be picky about his 'sales'.
EDIT2: OH SHIT! Just had a crazy idea come to me! Bob, what if the Soul Gems were also some sort of Mana Reactors? They feed off the ambient energies around them, producing refined "Magical" power for the girls. They do this taking the positive part of the energy - leaving the byproduct: negative energy... which is not used and is collected by the Soul Gem.
From there, MG's soul will become so corrupted by Negative Energy that she turns into a Witch... or she defeats a witch and offloads the negative energy into the resulting Grief Seed. Once a Grief Seed is filled to capacity, he eats it... which sends it off to his people so they can use it as they see fit.
Why his facial expression never changes
Why he seems to be cute without actually acting the part
Why he just doesn't get people
Why he seems to have ulterior motives
Why he frankly just doesn't give a damn
And last, but not least... Call me crazy, but while he seems perfectly acceptable, if a bit odd at first, prolonged exposure puts him firmly in the Uncanny Valley.
EDIT: in an odd way, Heisenburg's Uncertainty Principal applies here -
it's hard for Kyubei to tell if any wish she has was not influenced by
anything but her own desire. Also consider, he's got a 'business' to
run, so he can't afford to be picky about his 'sales'.
EDIT2: OH SHIT! Just had a crazy idea come to me! Bob, what if the Soul Gems were also some sort of Mana Reactors? They feed off the ambient energies around them, producing refined "Magical" power for the girls. They do this taking the positive part of the energy - leaving the byproduct: negative energy... which is not used and is collected by the Soul Gem.
From there, MG's soul will become so corrupted by Negative Energy that she turns into a Witch... or she defeats a witch and offloads the negative energy into the resulting Grief Seed. Once a Grief Seed is filled to capacity, he eats it... which sends it off to his people so they can use it as they see fit.