You never get the new class if you don't buy the new expansion (technically you do when the following expansion launches, but that's beside the point). It would definitely drive up interest in prepurchases if people immediately got access to a new class.
You could also argue the relative importance is arbitrary; some people would view earlier access to 7.3 allied races are more important than earlier access to a new class.
Someone link me the collector's edition leak, and I mean the picture not some text a dude could've come up with.