Funny you mentioned it. As you know, I was in the office until late on Monday. As I was walking home via Shattuck, I was wondering why many women were wearing more revealing than usual clothes until I remembered what day it was.
I believe many people allow themselves go wild because it affords them a release valve, to satisfy their baser (but natural) personality forces which they cannot exercise during the rest of the year (for example, blood, violence, and weaponry are also assimilated into costumes and identities). There should probably be a distiction drawn between those for whom irresponsible behavior is abnormal, but wish to dabble with it, and those for whom it is normal and, as you say, Halloween offers a convenient excuse to relish in it. The former, I feel, is psychologically healthy.