I'm surprised Sensei has hammered this one for years @ our dojo. "Aikido is proactive" is a favorite saying for him. As a matter of fact this is also quite often demostrated at Ikeda Sensei's seminars.
Keeping the dicussion at the level of principles, so without getting into the what if fighting scenarios, If you wait to apply technique you'll be setting yourself up for a struggle against the attacker when he/she's at his most powerful, that is centered. By meeting the attack , by taking the initiative you have a better chance of taking Nage's mind off the attack; to deliver atemi; to disrupt the attack's energy and then to take over Nage's energy and lead it where its safe for both.
This is not attacking someone , this is being 'there' so that the conflict is not a 50/50 chance.
In Randori we're taught never to stand still waiting for the attacker; we're taught to go out and offer a target to close in and force the attacker to go after you instead of you reacting to the attack; you can be overwhelemed very quickly if you let yourself get into reactive mode. Instead by being proactive you are always on the move and the attackers are pursuing you if at all. So you draw them into your technique and not the other way around.
Being proactive means not letting yourself be cornered into a wall, it means being ready always , it's having the initiative. It doesn't mean attacking first, it means having the intiative on the point of collision.
You are attacked perhaps first, but you are off the line of attack where it lands, BEFORE the attack hits you.
To me that's what Aikido is all about