THOSE of US who subscribe to the ideal of equal treatment under law for all citizens must agree that public money is well spent when it effectively reduces the disadvantages that the poor suffer under our present legal arrangements.  Judged by this standard, the legal assistance program sponsored by the Office of Economic Opportunity (OEO)-whose uncertain fate will soon be determined in Washington-has been a success. By providing free legal services, its 265 agencies with their more than 900 neighborhood offices have improved the ability of poor people to defend their interests. Whether as defendants or as plaintiffs, the poor can now have highly dedicated and skilled lawyers to represent them.