Only dragon knights (ie the main character) are wyverns, the rest of the dragons have 4 limbs in addition to wings. Larian thought the transformation would look better / be more logical, and the form itself be more closely associated with the human form if the human legs became legs and arms became wings.
I am assuming the PC version will have gamepad support. I think they said awhile ago both the PC and XBox interfaces were included in their test builds. Even if they remove the XBox interface as an option on the PC, I doubt they would remove gamepad support.
