First, where our goals are the same we are working together. That does not mean that we necessarily work under the same banner. I think that is important, and apparently so do theists who have aptly chosen the title Interfaith and invited the faithless to join them.

Second, on those areas where we disagree there is no possibility of working together, explicitly or otherwise. An obvious point, but still something I think that needed to be said.

So, if we work together already where we agree and we needn't do so under the banner of Interfaith, then when focusing on whether or not joining Interfaith efforts is a good thing, we ought perhaps to allow ourselves to be concerned at least a little with our own image. Or at least, having our bases covered, not be guilted into merging with causes managed under banners we detest for "greater goods" which we ourselves already serve.

The more meaningful discussion to be had from this question seems to me to be around whether or not it is best for atheists to disappear under the banner of Interfaith, and if that is what truly happens when atheists choose to work for or with Interfaith organizations.