I am going to write just a guess based on my own logic. I always thought that the wyvern has only four joints or two or maybe one on their wings so whatever. My guess is that the wyvern picks something up with only one or both of it's back legs (talons) because they have the most strength in their back limbs (or of course with their mouth, but that's obvious). The wings are an interesting topic though. If you did put the claws on the wings, then the wing structure might be a little different to the wings of a dragon without (the talons/fingers) but I do think that some wyverns do have the extra fingers on the wings. It depends mostly on who you ask. When I look at pictures most people depict wyverns with one claw, but I have seen a picture or two (I wasn't searching very long) with multiple finger-like claws. I could be wrong of course. If anybody would like to correct me, I would be happy to take the feedback. Sorry for so much text.