A couple of thoughts regarding the posts above this one...
TheBigA - I don't know what you mean by "ATC is a community project." Yeah, the host is there to get you from story to story, but I view them as anchors, just like on any other news program. There are correspondents filing stories, interviews conducted by the anchors, etc. Your phrasing makes it seem like the reporters are out covering what they want, and if they submit it, it will get on the air.
Of course the ATC anchors aren't executive producers or managing editors with final say for the show makeup. But I'm sure they attend editorial meetings and voice their opinions regarding where the show needs to go, that evening. I tend to think ATC sounds much more "put together" than Morning Edition, which is why I prefer it.
I noted above that ATC has had lots of odd voices filling in this summer. This is very rare for ATC, usually one of the trio will just solo host. Obviously this has been a trial for potential new hosts. Kelly McEvers (who sounds very good as a host) has been on this week with Robert Shapiro.
This then begs the question of why NPR needs so many hosts for their programs. They have 3 for Morning Edition, one each for Weekend Edition Saturday and Sunday, and another for ATC Weekend. The weekend hosts are rarely heard during the week - are they getting a full time salary for one broadcast a week? Meanwhile, with three hosts for ATC, they were on air seemingly randomly. No patterns or rhyme or reason to the days they are on air. How is the division of labor going to work with four hosts?
Finally, in regards to Michel Martin taking over weekend ATC, well, the life expectancy of hosts on that program is very short. Usually less than two years. Don't know why it can't keep someone in that spot. Arun Rath seemingly just took over the position, and a quick google does not reveal what is going to happen to him.