Give me a break.
To Dos are a fundamental feature of iCal, Entourage, Outlook, Exchange, the Palm OS, and pretty any personal organizer ever made. They're not complicated, unless you'd call email an unnecessary complication over simple pen and paper.
Writing out To Dos as simple text notes would be more complicated and tedious for people than using a proper To Do system. You couldn't schedule reminders. You couldn't move a To Do from one day to another, or prioritize them easily. They wouldn't sync effortlessly with your desktop computer, so you wouldn't have a copy of them no matter what screen you happened to be looking at (desktop, laptop, iPhone, etc.).
Anyway, Apple thought that To Dos were important enough to add into Leopard's new Mail program, so it's not too much to ask for iPhone support as well.