In tournaments the luxury of screwing around kind of drops away and you are left to ask this pivotal question among many others: Speed or power? Which do you need more of, given that you need both to one extent or another.
Power units allows you to disallow the enemy his speed in the first place. It is a form of defense really, because when you blast units down to size, you're lowering the inevitable damage you will take or their ability to position themselves the way they wished. You are taking away their resources BEFORE they can actually sacrifice them. Stranding an enemy who relies on speed can be very harmful to their chances, as these armies often only require one round to express that speed in order for it to be worth it. A bevy of firepower that steals the opportunity certainly matters. That relies on you gaining the initiative on the enemy however and as we know, that is at least in theory only happening 50% of the time! If you faced five fast armies in a day and went first just twice, would you lose the tournament simply for losing initiative? It surely emphasizes how important new models like Guilliman and oldies resurrected in new form such as Inquisitor Coteaz can be to a power minded list. Utility has a place even in those instances.
Some additional food for thought on the matter: Just look at the "power lists" below. What do you see?
Grey Knights: 3-5 DreadKnights with deep Striking allies
Chaos Daemons: Screamer Star with Belakor and summoning. Possibly even Magnus.