There is “X / week” column in some category styles, like the one used on meta. I think it’s kind of useless and only distracts users.
If for whatever reason you want to estimate how active a category is (which probably doesn’t happen often, and mostly useful for admins), you can just look at the dates in the Latest column, or open it and check some topics. (in @sam’s minimal theme there is also creation date in the topics list 
)
For now I just removed it via theme CSS
.category-list .topics {
display: none;
}
I think it should be optional and disabled by default.