Ubuntu's Debian technology already has the infrastructure for on-demand downloads and software installation, so why not monetize it?
Read the comment at ITManagement
But, if past incarnations of the idea are any indication, then the results are likely to be disappointing at best. For one thing, neither the free software community nor the software vendors care for the idea, so there's little market for it. For another, with the recent maturity of many pieces of free software, how many Ubuntu users will be so insistent on a brand name that they'll pay for functionality that they can get for free?