The voters do the vetting in the primaries....they have debates...just like they do now.
Incumbents have greater media access? No shit....but their voting record and their stances on the issues are still public record and there for everyone to see. It's called accountability. If the incumbent is doing a good job for his/her constituents....why SHOULDN'T he/she be re-elected? If he/she is not....then let his her PUBLIC record be their downfall.
What we have now is bullshit lies and half truths spewed every fucking 10 minutes on TV by anonymous douchebags with shady fucking names like "Americans for Prosperity" or Citizens for Progress"(to make it even). It's like Jerry Springer is running everyone's campaign.
To your last point? Because.....it will eliminate the stranglehold that monied interests have on our political process....and yes, I am talking left wing interests too. It will reduce corruption(I'm not sure you can ever eliminate it), and put people in office that actually WANT to SERVE this country and it's people.