i dont think anyone has said that additional characters will not be put in future betas, its just that its not top priority right now...
anyway, i think it would be easier to add characters after all the other features are put in and all bugs are sorted out
...otherwise the characters could be incompatible with future betas and would have to be remodelled, reskinned, reanimated, retested for bugs, etc...
in my oppinion i think it would be better for the team to finish all the gameplay features before adding extra characters.
also the modellers, animators and coders will be able to get the transformations done sooner if they dont try and add more characters like tien now...