There are two very different takes on this decision that McCandless makes.
Some people believe that his choice to not take up-to-date maps was foolish and that this act proves that he was an arrogant and ignorant fool. This single act is what essentially guarantees his death. Had he possessed a map, he would've been able to find his way to safety once he began to fall ill.
On the other side, some say that McCandless' choice was representative of the fact that he wanted the true, raw experience of being out in the wild. No maps meant that he would be forced to survive off his own wit and will.