Yeah, pretty much.
By giving it an active window, and a cd we allow the ability to be more powerful when we need it, and introduce the choice of when we feel it will best serve us.
For the wielder this means you can have a stronger thornmail effect, returning far greater damage than a 100% up time passive could...
For the opponent it means being able to bait or wait, and not being walled off from cleanup when the tank has lost the fight.
Personally - I've always felt it was a bit unfair that a marksman still couldn't clean up a tank with thornmail even after the fight was obviously won. I also thought the nerfs it received really negated any reason to buy the item. That's when I sorta though an active component could solve both problems.
Riot talks about wanting items to be bought on a per game basis... This could be the item you buy when you're against auto-attackers. Yi / Yasuo jump you? Hit the active. Long range Jinx winning fights while the team locks you down? Hit the active.
Buff it too, not a weak damage return that anyone can lifesteal through, but a harder damage return that makes it a bad call to throw autos at you. This can give tanks a bit more room to do their job without just giving them tons of armor. The game needs to be about options and plays, not raw stat wars.