 Being forced to use a "hybrid" time domain (filter bank) /frequency domain (MDCT) model to fit in with Layer II simply wastes processing time and compromises quality by introducing aliasing artifacts.MP3 has an aliasing cancellation stage specifically to mask this problem, but which instead produces frequency domain energy which must be encoded in the audio.This is pushed to the top of the frequency range, where most people have limited hearing, in hopes the distortion it causes will be less audible.