@ORJ
There is no guarantee that in a real-world failure scenario that you "have all the power you need even with an engine failure at T/O". The certification rules for determining single engine failure performance are specific, and not all-encompassing; there are failure scenarios that will result in losing more performance that is assumed.
So while there are very good reasons to leave the thrust where it is supposed to be - VMC not the least of those - someday someone will wish they had the full rated thrust, not the available derated max.
One example, is any failure which causes the failed engine to fail to windmill. The additional drag for a locked fan is considerable.