It is currently unknown as to why Google+ doesn't utilize hashtags. It is thought that they believe "Circles" solves the issue of content organization, however, Circles often fall short.
I believe Google+ should add a tagging functionality for posts and blend that into G+ Sparks results.
I have already circled people into circles based upon their profession or interests, however, I often find that that is not enough. I sometimes find myself wishing to browse for posts on a particular topic (such as marketing, SEO, or game design), but there is currently no functionality to help me with this.
Yes, I can browse through my "Marketing" circle for interesting discussions, but often times those users might be talking about their visit to a great sushi bar. This clutters up the feed with stuff I deem irrelevant to that circle.
An elegant solution would be allowing users to tag their posts based on the topic being discussed (just as one would tag the post for which circles to share it with). As such, if I tag a post with "SEO", it would show up in my followers' Sparks lists about "SEO" (if they have such a spark set up).
Or they could take a page from Tumblr and set their system up to be browsed by #hastag.