That's like saying, "I feel like if I needed to listen to music, I would just use my iPod that I spent $300 on". We bought the iPhone because it combines all of these things into one device, so having a scientific calculator would be a very nice touch (but obviously not vital at all right now, we have other issues).
I realize this is rather late in this discussion, but my company Sentry Apps, LLC has just recently made a businesss calculator for the iPhone and iPod Touch called BisCal. It includes the basic functions you mention, such as percent. So if that meets your needs, you can buy it at the app store.
We will be working on a scientific calculator soon called SciCal. So since you all had your different wishlist, and if you still want anything in particular, I encourage you to visit SciCal's forum and contribute to its features.
Thanks for expressing your opinions and wants. We app developers do listen.