There's two dynamics at work that you need to keep in mind, Peter. One is certainly effectiveness. Doing a technique "slow and soft" doesn't limit its effectiveness at all. If you can do it perfectly "slow and soft" then speeding it up and powering it up will happen when the need arises.
The other thing to consider is more basic. One of the big reasons for practicing Aikido "slow and soft" is for protection. Most of us want to still be doing Aikido when we're old and gray. If we're always doing thing as full speed and power, the risk of injury goes up substantially. Some dojos find that risk acceptable. Segal Sensei often comments that if you choose to train in his dojo, you will experience a serious (as in loaded up in the ambulance) injury before you reach shodan. That's not the sort of training environment I would enjoy, personally.