I also used to say that one should never leave a church if that church needs him/her in order to survive. But from God's point of view, no human being is indispensable. If he wants a weak church to keep going, he will supply the gifts that church needs. On the other hand, I have come to the view that it is not a tragedy when a tiny, stagnant, sick church folds up and dies. It is better for the members of such churches to be part of living, dynamic fellowships than to stay forever in a situation where they are constantly discouraged and, most likely, not being properly fed.
As someone who rarely, if ever, agrees with John Frame I find in this section from his work Evangelical Reunion (cf. Rev. Toby Brown's already linked discussion) a paragraph that is quite refreshing. I find myself presently at a time of much discernment and prayer as to my future in the PC(USA). As I think about this I am often troubled by further deepening the already wide gulf between the denominational landscape. What I personally struggle with is whether I am called to be like Jeremiah and others prophets (certainly not calling myself a prophet, that would be absurd) and stay in an increasingly apostate denomination preaching the gospel where many minds are shut to it or seek to follow the example of Matthew 10:14 and shake the dust from my feet and move on to another town.