Rick Perry’s presidential campaign may or may not be toast, but his prospects for hosting “Saturday Night Live’’ have risen considerably - and in politics, that’s not such a terrible thing. The benefits of going on late-night talk shows are well-known to candidates now: a larger and broader audience than they get from political shows, a chance to show the public their human side. But the not-so-dirty secret of late-night TV is that it’s often good for voters, too. It’s no shock that campaigns prefer the more collegial way of preparing for a late-night show, where the arc of an interview is laid out in advance, jokes are penned in collaboration with professional comedy writers, and the hosts have the freedom to indulge their curiosity, so long as the discussion seems interesting enough. In a Letterman apperance in 2004, Lehane recalls, then-presidential candidate Wesley Clark was treated to an 18-minute policy discussion that spanned two commercial breaks and barely contained a joke. “It was one of the most serious interviews in the entire campaign,’’ Lehane said.