I hate to say this, but I actually think it's more honest of them to have Feingold than Kerry if they're going to discuss this issue. Kerry is such a loser, and carries so much baggage from a nearly continuous stream of lies, that there is never an opportunity to honestly examine the point of view they want to get across. Feingold is a leftist freak, but he can put forward the message and allow viewers to examine it on the merits, not what they think of the messanger.
And with Feingold they get their own base to tune in, the moonbats. I bet they're even counting on getting good notices from Kos (it was probably part of the deal to turn the show over to Feingold).
The real problem for me is that, as you say, agenda trumps ratings. They are choosing to put forward Feingold as a more capable spokesman for the extreme leftist's point of view and they offer up no rebuttal. If the topic is something that conservatives are doing or saying they'll seek out the least effective spokesman (or better yet a RINO who disagrees with the position) plus they'll offer up three quarters of the show to rebuttals.
You know, during last Prez election time - politicians were running awy from Kos crowd, because of his "Screw Them" comments & pulling away ads from his site.
Same thing will happen this cycle too, and if you could believe they don't even need republicans for that.