It was definitely worth a read. A lot of what it said conflicted with the Social Anxiety book that I read, but the social anxiety book assumes you are wanting social relationships and can't get them, while the party of one book assumes you are happy with what you have.
Separate from the justification/validation aspect, just the research on movie characters and portrayals of the loner were worth it.
It is very us vs them in terms of loners vs non-loners which I thought was a bit hypocritical, but I was able to look past that and enjoyed it overall.