There was further elaboration of summoning in a really really really superb and heartbreaking story known as "Loraan's Smile" published in the anthology "Colors of Magic". In it, the artificer Feldon tried to learn about summoning, and part of that was getting to know a wizard for a few weeks. At the end of it, he tried to summon a replicate of the wizard. All the details were there, but they were distorted, some exaggerated, and others nearly non-existent. It comes from the fact that Feldon did not know the wizard really really well, so the image he conjured in his mind was influenced by his perception of the man and the purpose he needed.

I think it's because the changelings were inherently too goofy and... well, dumb as bricks.
The only thing we ended up with as a darker reflection were the mimics, but they were hardly the same thing when you get to the core of it.