That doesn't excuse the contradictions. One can't expect a new system to be planned out and solve every problem. Anarchism seems to offer a methodology for moving towards a fairer and just society which removes the contradictions inherent in a system of concentrated power.
Anarchism has its very own sloth of contradictions. If a society without power is established, someone will take power by force and/or guile and establish a new system of power that will not be anarchist in nature. Anarchism as a system is very instable -- read about the fate of the CNT during the spanish civil war for example.
Every system that is complex enough (societies are) will inevitable lead to contradiction. People do not act as in a model of society -- they will always find ways to break out of the model and subvert the ideal. This I think is the main reason left wing ideologies have lead to tyranny: If the citizen does not conform to the ideal of a model state, the state will need a secret-police apparatus to enforce this model.
Anarchism doesn't aim to establish a system without power. The example that a group would seek to rule through force and guile is very much true. In fact that group is already well established and indeed does use a militarised arm to enforce the model. Anarchism seeks to dismantle this, not enable it.
