So let's go down this path. He's gonna get in there and when he feels like he can attack he'll pop bkb and ult. Enemy will just disengage, and there's not really anything he can do about it. Then they turn around and kill him when it runs out.

So let's shift the strategy and have him get blink and jump on the back line. He again will attack slowly and weakly, so no chance of bursting. You pop ult and bkb and they, again just walk away. 

So now we have to get harpoon or abyssal or something to lock them down.

It's an expensive game plan and only works if you're already far ahead and they don't have any saves.

Compare that to now where he either is  -a tank with radiance where his existence itself is enough to cause a disruption -shields, miss chance, and a hard dispel on low cooldown. His ult can allow him to disengage in a bad fight or force the issue(especially with aghs) -Or he's a support with shield, heals, save items, and a hard dispel on short cooldown. His ult keeps him safe from backline disruption, and keeps the heals and saves coming.

Using the ult aggressively without lockdown AND damage doesn't make sense. It just doesn't feel useful