Lets be a little more precise. We both know we have the ability. You are addressing our level of competance.
But why should a lack of competance mean we should avoid exploiting this ability? Maybe we will all die if we don't. But maybe we will all die if we do. As you said we don't have the ability yet to decide which traits are good and which are bad.
There are some things in this world that if you can’t do them right they shouldn’t be done, especially not in a large scale. If you can’t build small dams compitently then you shouldn’t build the Hoover Dam, if you can’t build a five story building compitently you shouldn’t build the Empire State Building, and if you can’t breed German Shepherds compitenty you definetly shouldn’t try focused breeding on the whole human race. And the other side of trying to control evolution is that we know there’s already a natural system in place that can do it. With 6 billion people on the planet and growing we can be pretty sure we’re not aiming down a natural path towards extinction (unless we find a way to outbreed our food but that’s still a ways off inspite of what Toffler said), there’s no reason to try to guide our development when unguided development is going fine. The normal forces of natural selection have proven to be more than effective is sorting out the good traits and bad, if it ain’t broke don’t fix it, especially if you’re just guessing at what the fix is.
It doesn't matter. Eugenics, fundamentally, is no different than predation. The fact that we are aware of our role within the system, and the Tiger is not, is irrelevant form the perspective of evolution.