I agree with George that the strikes we use to practice in aikido initially seem artificial and unrealistic. But I would also characterize them as inefficient and exagerrated as well. However, in our dojo, when we practice against tsuki, the tsuki we attack with is the same as a traditional karate punch (reverse punch). Having practiced in another martial art prior to aikido (one which is considered one of the more "street effective" arts) I think there is another reason that the strikes we practice in aikido seem inefficient and exagerrated. This is so that the practitioner can understand the basics of the attack and the underlying movement. Once the aikidoist (or any martial artist) gets the basics down, THEN he can begin to work on responding to variations on those attacks (ie, more efficient and realistic strikes and attacks). Since I'm still working on getting the basics down in aikido, I can only support this argument based on my previous martial arts experience in observing my own and my previous students' development. But in the previous art I studied, the approach on practicing with such "exagerrated" attacks was the same.
By the way, even though it is not so common in aikido to practice techniques against roundhouse punches (probably the most common "street" attack), the body mechanics and movements associated with a yokomenuchi are almost identicle to a roundhouse punch. Just an observation on my part...