What's the point of lethean tea if irrigo's around

Maybe one of the writers can help me sort this out in my head, or at least a fan with a convincing theory. If irrigo is a known quantity in the Neath, why, if you wish something forgotten - would you bother with lethean tea? I’m aware long exposure to irrigo has detrimental effects, but it’s been shown that even short viewing can obscure long stretches of time from the mind. Is there a way to reconcile this discrepancy?

Lethean Tea is to Irrigo what a surgeon’s scalpel is to a battleax. If you’re aiming for a significant, but limited, removal of memories, you would choose the more precise tool. If you stayed in the Nadir long enough to forget the very reason you came to London, you would probably forget the way out soon after (or before).
edited by Rackenhammer on 12/15/2014

I thought Lethean tea specifically made you forget something important to you. (Ie your ambition) Whereas irrigo is just… everything.