well 1, there are plenty of abilities like this, sion's stun, xerath Q, pantheon ultimate. the point is to make it harder to use solo, so you can't just WEQR instakill everything. while it is still an aoe stun where if you get caught there's a good chance you're dead, it's harder to use. you have to be smarter about how you use it. leona E's an enemy, well that lasts about .25 seconds, and her stun is about 1 second, so if i WE the enemy right when i see leona E, my stun should land, forcing the enemy team to follow up or let poor little annie die via meteor shower. it's not really them being dumb enough to walk into it, it's about making it so it must be used wisely, and not just placed willy nilly every 12 seconds.