I beleive this is because avi for many years was traditionally image only. Both wav and avi have great quality, but are enormous. I just think it is older support. I think the psd format is also still old(2.5).You would still probably need to edit tho. Car is not going to give you all the track, mixer and editing features anyway.But since you can convert many things from avi it wouldn't be bad idea.

No need to think outside the box....
    Just make it invisible.