You'd have to be insane to tie a business or email to a privately-owned subdomain. It could get sold, the company could go under or any other type of mishap could happen.
P.S. co.uk is different as that was initially designed as that ccTLD base, and is still owned/controlled by the Brits, but simply delegated to a registry (Nominet UK). I also think .UK will eventually take over for the reasons stated above.