On reddit you could see karma breakdown by subreddit which was useful to see where people were most active.
I'd argue that doesn't really tell you where somebody is active. Most of my karma on Reddit was because I made literally one popular comment in a default subreddit. I could make a thousand comments elsewhere that would never see that kind of traction. You might get the impression that I was really into that subreddit even though I just saw it on /r/all and made a drive-by comment that struck a nerve.
I don't know that this is really a problem as such. Maybe people shouldn't care if other users get the wrong impression about which communities they're interested in, but ultimately where you make your reputation is probably more a function of where you're speaking to the largest audience. Unless you're avoiding larger communities completely, basically everybody will get most of their reputation from posts to the largest, most-visible communities rather than the smaller places where they may be spending as much or more time.