It already does. Go in settings and accounts I'm pretty sure it says when you were last on, on which device
I assume they mean so they can see if clannies / allies are active. While at first it sounds like a good idea, it would also mean we could track activity of people we wish to target. Which is fun for the attacker but not the defender
This could potentially be useful if instead of showing the last date active, profiles would show a special symbol or spell if the account has been inactive for more than 7 days. This would let you know if your clan mates or potential allies are inactive without being vulnerable to stripping and/or farming while asleep.