Any sort of MPEG/MP3 support is usually not available in beta versions due to licensing fees. Even though it's a free beta version, SonicFoundry would still owe the licensing fee to the Motion Picture Experts Group. For this reason they wait until people are paying for the shipping version before adding MPEG/MP3 features.
I think that it also compromises the product's integrity. Who want's their product associated with compromised sound quality sorced from an MP3 ?
That being said, they can't have a 'music taste and recording quality' filter on the front end either, so it is inevitable it will be included on release.... ;-)