I haven't really been paying attention to the tags.
One thing is that only the very simplest problems will fall under a singular theme. The more difficult problems will often have a variety of themes, especially when you consider sub-variations.
As I go through the problems, maybe I'll try to start voting for appropriate tags. I guess if there's multiple themes, I'll vote for the predominant theme if there is one.
The tags will really come into their own with the problem search and custom problem sets where users can create problem sets based on tags, for example a problem set that includes all forks (or refining further to say a problem set with all forks and ratings above 2000). At the moment there are probably not enough tags done to make this super useful but I hope that changes over time.
The problem search and custom problem sets are paid subscription only features, but hopefully tags are also useful to non-paying users, especially beginning level players who might need signposts to help better understand a problem.
I agree that many problems have multiple motifs, since the recent UI update users have been able to vote for multiple tags per problem (previously there was only one vote per user per problem), so if you see multiple themes don't hesitate to tag all of them.
Regards,
Richard.