Saves some lines on the member card, but the suggestion is mostly because the user's username is already at the top of the member card or profile, so it seems kind of redundant to have it again.
In regard to saving lines on the member card, if you have a long username, then your activity gets wrapped onto the next line.
Short Username Example:
Long Username Example:
However, if you change X was last seen to Last Activity, it doesn't matter how long the username is, because Last Activity will always be 13 letters compared to varying usernames. For example, Amaury was last seen is 20 letters and my example username along with the was last seen is 35 letters.
While I know this isn't difficult to do and can be done by simply editing the phrase, I think it'd be nice if it were like this by default.
Again, though, the suggestion is mostly because it seems unnecessary to have the username there twice.