For it to work right, it would have to be an honest devil's advocate. while there are plenty of devils around Washington, there have been no sightings of honest devils in recent history. Hence, no honest devil's advocates.

Getting someone with a party as well as ideological stake against you and letting them in on plans is going to cause way too much trouble.

OTOH, you might reasonably hope they'd have a bi-partisan-loathing libertarian about. Somehow I doubt it; maybe they've tried, but the libertarians always give up and run away screaming.