My understanding is that membership in the IMF is strictly voluntary. Therefore one presumes that ever member country believes that membership is better than non-membership. Of course many developing world countries have a large debt load hanging over them and are thus like an indebted individual in that they do not have many easy options. In this case the problem is not the IMF but the pre-existing debt. The moral fault lies with those who ran up the debt (typically past presidents or dicators) and those who refuse to retire the debt (typically Western banks and governments). My understanding is that by keeping bankrupt countries afloat the IMF is part of the solution, not part of the problem. I also notice that the IMF is explicitly attacking the issue of indebtedness (IMF debt relief initiative).
If the IMF disappears, the debt will not. Conversely, if the debt goes away then any country can choose to participate or not participate in the IMF with impunity. Therefore I feel that advocates for the poor should focus efforts on retiring the debt, not on destroying the IMF. Perhaps the IMF is well-managed and perhaps not but it is surely not hurting the world's poor in and of itself. It seems to me that it is merely a stand-in for those who hold the debt and refuse to retire it. Therefore why would advocates of the poor attack the IMF rather than the debt-holders? Would Argentina be better off if it had bilaterally (failed to!) re-negotiate its debt with every other country and bank in the world?