I don't think any technique in karate is meant to be perfect and useful for each and every situation. Of course, a good shuto uchi to the neck and nearly everyone is down, but you could argue that you can't always use a shuto.
Kihon, as Zach and Dan said, is there to teach you principles. I guess you could say these techniques are to give you an idea of what you can do when in a fight. As you practice gyaku tsuki, you develop your hip movement, which is useful not only for the reverse punch, but also for nearly every technique in karate, where the power comes from the whole body, and not just from the arms or the legs - so you actually are learning a lot of karate just with one punch.
Perhaps someday you get into a fight (Pray God you don't

) and, after doing some taisabaki, you find yourself sideways to your opponent, with his ribs full open to you. Heck, I say a gyaku tsuki is good for that. Then run - or, as neko456 prefers, kick him to the ground and destroy him - poor fella!
When I think something is useless, I stop doing it. Would you stop training gyaku tsuki?