Many users use Twitter as a communication service between a tight circle of friends and associates, rather than a public conversation with the entire world. These users still may have hundreds of followers, but all their followers have been “vetted” and are privy to the user’s private feed.

Hashtags are completely valid for these users, they’re just used differently. Public users’ feeds can be found by searching hashtags, private users use hashtags as a one-way link from their feed out to an idea in the greater Twitterverse.

So all those private feeds you complain about, they’re only useless to you, someone on the “outside”, like they’re supposed to be.