This Observer piece flagged on HuffPo explains why he shouldn't give them that platform but it doesn't explain why he does.
Here's why he does. It's not because Russert secretly supports Hillary or Fred, obviously. It's because Carville and Matalin are his kinda people. Homegrown Americans, rough hewn and down to earth, no academic airs or ideological certainties to interfere with their horse race realism, no bothersome beliefs to dilute the flow of wit and fellow feeling among the only folks who really get it in this town--the pros at the top.
Russert thinks, hey, I got a free half hour here, let's bring in the cue-ball and his sharp-tongued bride, kick back and have a little fun -- on the show, just the way we do in the saloon. Give the regular folks a taste of what it's like to be in the coolest clique of all.
Russert doesn't care about their bias toward Hillary or Fred. Neither do Carville and Matalin, actually. It just happens that these particular pros have thrown in with these particular candidates in an endless game that transcends all issues. Isn't that the whole message of their much-ballyhooed marriage? The personal trumps the political? These people are way past truly caring about anything substantive. All they really care about is the play.
Why? Because they are the players.
So why does Russert give them that platform? They're pals, that's why. It's as simple as that.